Disable use of weak in threads_posix.h on Cygwin
Disable use of weak in threads_posix.h on Cygwin

Weak doesn't work the same on PE/COFF as on ELF, they are only weak
references.  Specifically, since nothing else pulls in the object which
contains pthread_mutexattr_init() (and coming from the C library, that is
the only thing that object contains), means that it ends up as 0

Signed-off-by: Jon Turney <45846cba2ea093796a8b1c25d70d679d8107179f@dronecode.org.uk>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler