Add "Posix" global version conditional for Emscripten targets

[ryuukk]

I just had this thought after seeing this: https://forum.dlang.org/post/[email protected]

Haven't tested yet, but i submit the PR otherwise i'll forget

And i can't compile ldc from source:

cmake -G Ninja ../ -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=$PWD/../install-ldc

/usr/bin/ld: cannot find -lLLVMSymbolize: No such file or directory