Check for libc++ and libstdc++

[elde-n]

By default all platforms assume the c++ standard library to use, even if it doesn't exist on the machine it will try to link with it, on linux where you have user choice you should at least check that the files: libc++.so or libstdc++.so exist

[NobodyXu]

Hmmm...maybe the best way to check is to try compile it with the specified stdlib?

That's indeed possible and might be reasonable c. @madsmtm

[madsmtm]

Please see cc::Build::cpp_link_stdlib.

@elde-n, could you explain why the options in there do not fit your use-case?

[elde-n]

Exporting CXXSTDLIB=c++ as a user is fine, but why not just automate the process?

[NobodyXu]

Do you want cc to pickup environment variable CXXSTDLIB @elde-n ?

I think adding an env var is ok

[ReillyBrogan]

We hit this same issue on AerynOS which uses libc++ as the default c++ stdlib.

[NobodyXu]

I would welcome any PR for this, it probably need to try launching cc/cxx with different cxx value