Revert "configure: require pthreads for POSIX builds"
Revert "configure: require pthreads for POSIX builds"

This reverts commit 50714cec2b50c7836841c09f04bfe875de00ae1d.

Not meant to go in yet. Lacking review.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler