glsl: Link glsl_compiler with pthreads library.

glsl: Link glsl_compiler with pthreads library.

Fixes the following build error on OpenBSD:

./.libs/libglsl.a(builtin_functions.o)(.text+0x973): In function `mtx_lock':
../../include/c11/threads_posix.h:195: undefined reference to `pthread_mutex_lock'
./.libs/libglsl.a(builtin_functions.o)(.text+0x9a5): In function `mtx_unlock':
../../include/c11/threads_posix.h:248: undefined reference to `pthread_mutex_unlock'

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Brian Paul <brianp@vmware.com>