reinvestigate c++ standard library behavior of Clang

[SimeonEhrig]

During writing the documentation about the standard library selection in Clang for PR #1695 I used the compiler flag --gcc-toolchain to select the libc++ version for Clang. This does not work, if the GCC compiler is installed under /usr/ like debian's apt is doing it.

I found another flag (--gcc-install-dir) and should check, if this is fixing the problem. https://discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091/1

[SimeonEhrig]

The feature --gcc-install-dir is not implemented yet: https://reviews.llvm.org/rG23e4299261d9