ax_pthread/clang: move -pthread to LIBS

clang also requires linking with -pthread to use pthreads.
FFmpeg linking is broken otherwise due to undefined symbols.

Signed-off-by: Christopher Degawa <ccom@randomderp.com>

m4/ax_pthread.m4

@@ -261,8 +261,8 @@ if test "x$ax_pthread_clang" = "xyes"; then
         # -pthread does define _REENTRANT, and while the Darwin headers
         # ignore this macro, third-party headers might not.)
 
-        PTHREAD_CFLAGS="-pthread"
-        PTHREAD_LIBS=
+        PTHREAD_CFLAGS=
+        PTHREAD_LIBS="-pthread"
 
         ax_pthread_ok=yes