Undefine the four JSON_HEDLEY_* macros that leak after including json.hpp (#5475)

* Undefine the four JSON_HEDLEY_* macros that leak after including json.hpp

include/nlohmann/detail/macro_unscope.hpp includes hedley_undef.hpp to
#undef every JSON_HEDLEY_* macro so none of them leak into the including
translation unit. Four macros were missing from that list and therefore
stayed defined after #include <nlohmann/json.hpp>:

- JSON_HEDLEY_PRAGMA
- JSON_HEDLEY_PREDICT_TRUE
- JSON_HEDLEY_PREDICT_FALSE
- JSON_HEDLEY_CLANG_HAS_DECLSPEC_ATTRIBUTE

hedley_undef.hpp is generated (via `make update_hedley`) by grepping
hedley.hpp for its own internal `#undef JSON_HEDLEY_X` redefinition
guards. JSON_HEDLEY_PRAGMA/PREDICT_TRUE/PREDICT_FALSE have no such guard
in upstream Hedley, so they were never picked up. The guard for
JSON_HEDLEY_CLANG_HAS_DECLSPEC_ATTRIBUTE also has an upstream typo
(`JSON_HEDLEY_CLANG_HAS_DECLSPEC_DECLSPEC_ATTRIBUTE`), so hedley_undef.hpp
was undefining the wrong (never-defined) name.

Fixes:
- include/nlohmann/thirdparty/hedley/hedley_undef.hpp: corrected the
  DECLSPEC_ATTRIBUTE typo and added the three missing #undef lines,
  keeping the file's alphabetical ordering.
- Makefile (update_hedley target): changed hedley_undef.hpp generation to
  extract macro names directly from every `#define JSON_HEDLEY_...` in
  hedley.hpp instead of from existing `#undef` guards, so a future
  `make update_hedley` run undefines every macro Hedley actually defines,
  even ones without a pre-existing redefinition guard. This was not run
  in this PR (it would also pull in an unrelated upstream Hedley sync);
  hedley_undef.hpp was hand-patched instead and single_include was
  regenerated with `make amalgamate`.
- tests/src/unit-no-macro-leak.cpp: new regression test (picked up
  automatically by tests/CMakeLists.txt's existing unit-*.cpp glob) that
  includes json.hpp and then #ifdef/#error-checks every JSON_HEDLEY_*
  macro name, so any future leak of any of the 151 vendored macros fails
  the build, not just the four fixed here.

Fixes #5408.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Derive the JSON_HEDLEY_* leak-check test from hedley.hpp at build time

tests/src/unit-no-macro-leak.cpp previously hardcoded a static list of
~151 #ifdef/#error checks, one per JSON_HEDLEY_* macro name known at the
time it was written. That list would silently go stale the next time
`make update_hedley` pulls in a vendor update that adds, removes, or
renames a macro, since nothing would force it to be regenerated.

Add cmake/scripts/gen_hedley_undef_check.cmake, which derives the full
list of JSON_HEDLEY_* macro names directly from
include/nlohmann/thirdparty/hedley/hedley.hpp:

- tests/CMakeLists.txt uses it (MODE=checks) to (re)generate
  hedley_undef_checks.inc at configure and build time, and wires the
  generating custom target as a dependency of the test-no-macro-leak_cpp*
  targets so it can never build against a stale copy. unit-no-macro-leak.cpp
  now just #include-s the generated file inside its TEST_CASE instead of
  carrying the checks itself.
- The Makefile's `update_hedley` target now delegates hedley_undef.hpp
  generation to the same script (MODE=undef, new `update_hedley_undef`
  target), so the vendored header, the generated #undef list, and the
  generated test checks are all derived from the same extraction logic and
  cannot drift apart.

This mirrors the approach taken independently in #5415 for the same
issue (#5408), credited there to a self-regenerating mechanism that
"can never drift again" -- ported into this branch instead of the
static list originally proposed here.

Verified with a local CMake configure + build + ctest, both against
include/ (JSON_MultipleHeaders=ON) and against the amalgamated
single_include/nlohmann/json.hpp (JSON_MultipleHeaders=OFF), and by
temporarily deleting a #undef line from hedley_undef.hpp to confirm the
generated test actually fails on a real leak.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix REUSE compliance failure in gen_hedley_undef_check.cmake

The generated file's embedded banner contains the literal text
'SPDX-License-Identifier: MIT' as part of the *content* being written
to hedley_undef.hpp, not as this .cmake script's own REUSE header (it
is already covered by the blanket 'Files: *' rule in .reuse/dep5).

The reuse tool matched that embedded line as an SPDX tag for the
script itself and failed to parse the trailing 'MIT\n")' as a valid
SPDX License Expression, breaking ci_reuse_compliance. Wrap the
embedded banner in REUSE-IgnoreStart/REUSE-IgnoreEnd comments, as
recommended by the tool's own diagnostic output.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: pretty clean ChangeLog.md release
+.PHONY: pretty clean ChangeLog.md release update_hedley update_hedley_undef
 
 ##########################################################################
 # configuration
@@ -41,6 +41,8 @@ all:
 	@echo "fuzz_testing_ubjson - prepare fuzz testing of the UBJSON parser"
 	@echo "pretty - beautify code with Artistic Style"
 	@echo "run_benchmarks - build and run benchmarks"
+	@echo "update_hedley - download Hedley and regenerate hedley.hpp / hedley_undef.hpp"
+	@echo "update_hedley_undef - rebuild hedley_undef.hpp from the JSON_HEDLEY_* #define names in hedley.hpp"
 
 
 ##########################################################################
@@ -241,11 +243,24 @@ update_hedley:
 	rm -f include/nlohmann/thirdparty/hedley/hedley.hpp include/nlohmann/thirdparty/hedley/hedley_undef.hpp
 	curl https://raw.githubusercontent.com/nemequ/hedley/master/hedley.h -o include/nlohmann/thirdparty/hedley/hedley.hpp
 	$(SED) -i 's/HEDLEY_/JSON_HEDLEY_/g' include/nlohmann/thirdparty/hedley/hedley.hpp
-	grep "[[:blank:]]*#[[:blank:]]*undef" include/nlohmann/thirdparty/hedley/hedley.hpp | grep -v "__" | sort | uniq | $(SED) 's/ //g' | $(SED) 's/undef/undef /g' > include/nlohmann/thirdparty/hedley/hedley_undef.hpp
 	$(SED) -i '1s/^/#pragma once\n\n/' include/nlohmann/thirdparty/hedley/hedley.hpp
-	$(SED) -i '1s/^/#pragma once\n\n/' include/nlohmann/thirdparty/hedley/hedley_undef.hpp
+	$(MAKE) update_hedley_undef
 	$(MAKE) amalgamate
 
+# Rebuild hedley_undef.hpp from every JSON_HEDLEY_* name that hedley.hpp
+# #defines. Hedley does not #undef all of its public macros internally (see
+# #5408), so grepping those #undef lines misses names such as
+# JSON_HEDLEY_PRAGMA. cmake/scripts/gen_hedley_undef_check.cmake is the
+# single source of truth for this extraction (tests/CMakeLists.txt uses the
+# same script, in MODE=checks, to generate the matching leak-check test), so
+# the vendored header, the generated #undef list, and the regression test
+# cannot drift apart.
+update_hedley_undef:
+	cmake -DHEDLEY_HPP=include/nlohmann/thirdparty/hedley/hedley.hpp \
+	      -DOUTPUT=include/nlohmann/thirdparty/hedley/hedley_undef.hpp \
+	      -DMODE=undef \
+	      -P cmake/scripts/gen_hedley_undef_check.cmake
+
 ##########################################################################
 # serve_header.py
 ##########################################################################