c11/threads: create mutexattrs only when needed
c11/threads: create mutexattrs only when needed

If the mutexattrs are the default one can just pass NULL to
pthread_mutex_init. As the compiler does not know this detail it
unnecessarily creates/destroys the attrs.

Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler