Allow cpp_std=c++23 with gcc-11.

[bredelings]

This PR allows using cpp_std=c++23 with gcc-11 as per issue #12291.

Currently meson requires GCC version >=12.2.0 for cpp_std=c++23. However, this seems incorrect, as gcc-11 also supports -std=c++23. Obvious c++23 support isn't complete in gcc-11, but it isn't complete in gcc-12 either.

The commit adding support for -std=c++23 was done in January 2021, several months before the gcc-11 release, so 11.0.0 seems fine.

• https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=78739c2df788ee5c868d998a6333d453317d8711

In GCC-11, three new features are enabled by this flag:

• https://gcc.gnu.org/projects/cxx-status.html
• Literal Suffix for (signed) size_t
• Make () more optional for lambdas
• Allow Duplicate Attributes

[tristan957]

I guess technically this could be a 1.2.3 issue. I will let someone else make that call.