Configure G++ to support the `-stdlib=` option

[cpplearner]

GCC recently added a -stdlib= option, which makes it easier to use libc++ with g++. But this option is only enabled when GCC is configured with the --with-gxx-libcxx-include-dir= option.

It would be nice if compiler-explorer could support this option.

[partouf]

Not exactly sure how we can make this work and also be practical for the website. I guess we could set it to clang-trunk for gcc trunk, but if this gets into a release it will be a bit less flexible than the --gcc-toolchain option the clangs have - unless they decide to include something similar.

[iains]

you mean if GCC added a --with-libcxx-install= (I guess that would make the --with-gxx-libcxx-include-dir= redundant). It needs some careful thinking about to avoid conflicts with sysroots and to work as expected in both native and cross toolchains.

NOTE: one can enable -stdlib= by setting --with-gxx-libcxx-include-dir without specifying a path; in that case the GCC configure will:

1. enable -stdlib=
2. set the path for the libcxx headers to <compiler install path>/include/c++/v1.
3. assume that libc++ (and libc++abi where that is needed) will be found on default linker paths

It is then reasonably convenient to symlink <compiler install path>/include/c++/v1 to the correct headers.

[iains]

I guess the main thing to watch out for is that sometimes the two toolchains implement builtins that support the library in a different sequence - so that there could be cases where the library wants a builtin that's not yet defined.