Use a proper compiler test for constexpr support

Use a proper compiler test for constexpr support

After GCC 15, C23 became the default version. [1]

Add a proper compiler test for constexpr support to ensure that GCC 15+
invoked with `c_std` set to `none` does not erroneously use the
constexpr shim.

This change does not affect the build in its default configuration
(which uses GNU C11); it is only meant to prepare the build system for
adding C23 elements to the code.

[1]: https://gcc.gnu.org/gcc-15/changes.html