We previously were defining _BSD_SOURCE right before including getopt.h.
However, on mingw-w64, getopt.h is also transitively included by
unistd.h, and unistd.h can be transitively included by many headers
(recently, by some libc++ headers).
Therefore, to be safe, we need to define _BSD_SOURCE before including
any header. Thus do this in CMake.
This fixes https://github.com/llvm/llvm-project/issues/76050.
f70b229e96