Check in CI that Meson and pkg-config offer the CMake options

tools/check_build_options/check_build_options.py takes the compile
definitions of the CMake target as the reference and checks that the
CMake pkg-config block, meson_options.txt (name and default), meson.build,
and the Meson section of the package manager docs have each option with
the same definition and condition. It runs as make check_build_options
in the ci_meson_install job, so a new CMake option cannot be forgotten
in the Meson build again.

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

tools/check_build_options/README.md

@@ -0,0 +1,21 @@
+# check_build_options
+
+Checks that the Meson build and the pkg-config files offer the same options as the CMake target, so that a new CMake
+option is not forgotten in one of them.
+
+The compile definitions of the CMake target (`target_compile_definitions` in [`CMakeLists.txt`](../../CMakeLists.txt))
+are the reference. When you add an option there, also add it to
+
+- the pkg-config block in `CMakeLists.txt` (`NLOHMANN_JSON_PKGCONFIG_CFLAGS`),
+- [`meson_options.txt`](../../meson_options.txt), named without the `JSON_` prefix and with the same default,
+- [`meson.build`](../../meson.build) (`json_defines`), and
+- the list of Meson options in
+  [`docs/mkdocs/docs/integration/package_managers.md`](../../docs/mkdocs/docs/integration/package_managers.md).
+
+Run the check with
+
+```shell
+make check_build_options
+```
+
+It needs only Python 3 and runs in the `ci_meson_install` CI job.