CMake: depend on libc++ when compiling with Clang

[robUx4]

libstdc++ is a GCC thing.

Issue

When compiling with llvm-mingw the .pc file references -lstdc++ which doesn't exist as it's a GCC thing.

Solution

CMake can tell which compiler is used.

Rather than hardcoding thing it might be better to just use CMAKE_CXX_IMPLICIT_LINK_LIBRARIES: https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_IMPLICIT_LINK_LIBRARIES.html

[mav-intel]

Hi @robUx4 Thanks for identifying this issue! A more general solution seems to be to rely on the compiler or CMake to include the library by default instead of calling it out explicitly. This approach is implemented in 80ea0a5