Disable numeric_limits specialization for GCC-14

[mborland]

Closes: #992

[mborland]

I built GCC from source, test_cstdfloat and the minimal reproducer from the example both pass fine with this PR.

[jzmaddock]

Just checking.... does libstdc++3 encase numeric_limits<__float128> inside a #if? They often do, in which case it might be better to check for whatever internal macro they are using rather than assuming gcc-14 unconditionally supports this.

[mborland]

It does have an #if block, and the diff is here. The two macros they use are already wrapped into defining BOOST_MATH_USE_FLOAT128.

[jzmaddock]

Ah, nothing useful there then, this all looks good to me then.

[jwakely]

This check seems wrong. The new numeric_limits<__float128> specialization will be enabled when using new libstdc++ headers with Clang, which does not define __GNUC__ to 14 (it can be told to use any value, but by default it defines it to 4).

[jwakely]

Checking _GLIBCXX_RELEASE < 14 would be more correct, as that is determined by the libstdc++ headers, not by the compiler including them.

[jwakely]

It does have an #if block, and the diff is here. The two macros they use are already wrapped into defining BOOST_MATH_USE_FLOAT128.

FWIW the current code uses slightly different conditions:
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/std/limits;h=ec0b7a1ca7bf38b7440b6e94c692cd5937f1bf23;hb=HEAD#l2084

That was changed by https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=6261d10521f9fdc2a43d54b4dc365025288aa8e9