[committed] libstdc++: Guard use of new built-in with __has_builtin

Tested x86_64-linux, pushed to trunk. -- >8 -- Another case where I forgot that non-GCC compilers don't have this built-in yet. libstdc++-v3/ChangeLog: * include/bits/invoke.h (__invoke_r): Check __has_builtin(__reference_converts_from_temporary) before using built-in. -

[committed] libstdc++: Guard use of new built-in with __has_builtin

This fixes libstdc++ for Clang and other non-GCC compilers. Tested powerpc64le-linux. Pushed to trunk as r13-2954-gdf7f2736509cfe. -- >8 -- I forgot that non-GCC compilers don't have this built-in yet. For Clang we could do something like the check below (as described in P2255), but for now I'm