gallium: Add pthread compiler flags

There are places in the code where we use pthread_* functions,
but forgot to add corresponding bits onto the compiler command
line. Because of that the build fails:

  CCLD     virgl_test_server
../src/.libs/libvirglrenderer.so: undefined reference to `pthread_sigmask'
../src/.libs/libvirglrenderer.so: undefined reference to `pthread_join'
../src/.libs/libvirglrenderer.so: undefined reference to `pthread_create'
../src/.libs/libvirglrenderer.so: undefined reference to `pthread_mutexattr_init'
../src/.libs/libvirglrenderer.so: undefined reference to `pthread_mutexattr_destroy'
collect2: error: ld returned 1 exit status

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/gallium/auxiliary/Makefile.am

@@ -9,10 +9,14 @@ AM_CFLAGS = \
 	-I$(top_srcdir)/src/gallium/auxiliary/util \
 	-I$(top_srcdir)/src/gallium/auxiliary/ \
 	$(GALLIUM_CFLAGS) \
+	$(PTHREAD_CFLAGS) \
 	$(VISIBILITY_CFLAGS)
 
 AM_CXXFLAGS = $(VISIBILITY_CXXFLAGS)
 
+AM_LIBADD = \
+	$(PTHREAD_LIBS)
+
 libgallium_la_SOURCES = \
 	$(C_SOURCES) \
 	$(GENERATED_SOURCES) \