configure: require pthreads for POSIX builds
configure: require pthreads for POSIX builds

This has been an implicit rule for building mesa for a long time. Let's
make it official and just bail out at configure time. This way we can
cleaning up some of our glx code.

Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler