macOS: Don't link lib{pthread,m} as platform libs

[kinke]

A little test wrt. #3901. But -lm is part of llvm-config --system-libs, with both official LLVM 16 and 17. And this then leads to ld: library 'm' not found unless enforcing /usr/bin/c++ as linker driver.

[JohanEngelen]

Is there something I can test for you locally? (perhaps just the linker-gcc.cpp change?)

[kinke]

Hmm, I don't know. I just hate that our CMake build doesn't work out of the box with recent Xcode, requiring that dead-ugly -DLDC_LINK_MANUALLY=OFF -DD_COMPILER_FLAGS=-gcc=/usr/bin/c++ workaround.