link mesautil with pthreads

../../src/util/.libs/libmesautil.a(libmesautil_la-u_queue.o): In function `u_thread_setname':
/builddir/build/BUILD/mesa-17.3.1/src/util/../../src/util/u_thread.h:66: undefined reference to `pthread_setname_np'
../../src/util/.libs/libmesautil.a(libmesautil_la-u_queue.o): In function `thrd_join':
/builddir/build/BUILD/mesa-17.3.1/src/util/../../include/c11/threads_posix.h:336: undefined reference to `pthread_join'
../../src/util/.libs/libmesautil.a(libmesautil_la-u_queue.o): In function `u_thread_create':
/builddir/build/BUILD/mesa-17.3.1/src/util/../../src/util/u_thread.h:48: undefined reference to `pthread_sigmask'
../../src/util/.libs/libmesautil.a(libmesautil_la-u_queue.o): In function `thrd_create':
/builddir/build/BUILD/mesa-17.3.1/src/util/../../include/c11/threads_posix.h:296: undefined reference to `pthread_create'
../../src/util/.libs/libmesautil.a(libmesautil_la-u_queue.o): In function `u_thread_create':
/builddir/build/BUILD/mesa-17.3.1/src/util/../../src/util/u_thread.h:50: undefined reference to `pthread_sigmask'
/builddir/build/BUILD/mesa-17.3.1/src/util/../../src/util/u_thread.h:50: undefined reference to `pthread_sigmask'
../../src/util/.libs/libmesautil.a(libmesautil_la-u_queue.o): In function `call_once':
/builddir/build/BUILD/mesa-17.3.1/src/util/../../include/c11/threads_posix.h:96: undefined reference to `pthread_once'
../../src/util/.libs/libmesautil.a(libmesautil_la-u_queue.o): In function `u_thread_get_time_nano':
/builddir/build/BUILD/mesa-17.3.1/src/util/../../src/util/u_thread.h:84: undefined reference to `pthread_getcpuclockid'
collect2: error: ld returned 1 exit status

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Igor Gnatenko <ignatenko@redhat.com>
(cherry picked from commit 23ce1680)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104818