docs: use HTTPS for the astyle and cppcheck links in README (#5351)

* docs: use HTTPS for the astyle and cppcheck links in README

Both links were still `http://`. `astyle.sourceforge.net` serves HTTPS
directly; `cppcheck.sourceforge.net` redirects to
`https://cppcheck.sourceforge.io`, which is also the URL already used in
`docs/mkdocs/docs/community/quality_assurance.md`, so the redirect is
skipped here.

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

* CI: suppress -Wc2y-extensions for Clang

Clang 22.1 (now shipped by silkeh/clang:latest) diagnoses __COUNTER__ as a
C2y extension, and does so in C++ mode as well. Under -Weverything -Werror
this breaks every ci_test_clang_cxx* / ci_test_clang_libcxx_cxx* target,
independently of the code under test.

The library itself does not use __COUNTER__; all diagnostics originate in
vendored Doctest (DOCTEST_ANONYMOUS, used by TEST_CASE and SECTION).

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

---------

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

README.md

@@ -1801,13 +1801,13 @@ The library itself consists of a single header file licensed under the MIT licen
 - [**amalgamate.py - Amalgamate C source and header files**](https://github.com/edlund/amalgamate) to create a single header file
 - [**American fuzzy lop**](https://lcamtuf.coredump.cx/afl/) for fuzz testing
 - [**AppVeyor**](https://www.appveyor.com) for [continuous integration](https://ci.appveyor.com/project/nlohmann/json) on Windows
-- [**Artistic Style**](http://astyle.sourceforge.net) for automatic source code indentation
+- [**Artistic Style**](https://astyle.sourceforge.net) for automatic source code indentation
 - [**Clang**](https://clang.llvm.org) for compilation with code sanitizers
 - [**CMake**](https://cmake.org) for build automation
 - [**Codacy**](https://www.codacy.com) for further [code analysis](https://app.codacy.com/gh/nlohmann/json/dashboard)
 - [**Coveralls**](https://coveralls.io) to measure [code coverage](https://coveralls.io/github/nlohmann/json)
 - [**Coverity Scan**](https://scan.coverity.com) for [static analysis](https://scan.coverity.com/projects/nlohmann-json)
-- [**cppcheck**](http://cppcheck.sourceforge.net) for static analysis
+- [**cppcheck**](https://cppcheck.sourceforge.io) for static analysis
 - [**doctest**](https://github.com/onqtam/doctest) for the unit tests
 - [**GitHub Changelog Generator**](https://github.com/skywinder/github-changelog-generator) to generate the [ChangeLog](https://github.com/nlohmann/json/blob/develop/ChangeLog.md)
 - [**Google Benchmark**](https://github.com/google/benchmark) to implement the benchmarks

cmake/clang_flags.cmake

@@ -5,6 +5,9 @@
 # -Wno-extra-semi-stmt            The library uses assert which triggers this warning.
 # -Wno-padded                     We do not care about padding warnings.
 # -Wno-covered-switch-default     All switches list all cases and a default case.
+# -Wno-c2y-extensions             Clang 22.1 diagnoses __COUNTER__ as a C2y extension, also in
+#                                 C++ mode. The library does not use __COUNTER__; the warnings
+#                                 all come from vendored Doctest (SECTION/TEST_CASE macros).
 # -Wno-unsafe-buffer-usage        Pervasive: the library's own low-level numeric/buffer code
 #                                 (to_chars, serializer, lexer, binary reader/writer, input
 #                                 adapters, json_pointer) plus vendored Doctest itself (~208
@@ -20,5 +23,6 @@ set(CLANG_CXXFLAGS
     -Wno-extra-semi-stmt
     -Wno-padded
     -Wno-covered-switch-default
+    -Wno-c2y-extensions
     -Wno-unsafe-buffer-usage
 )