move pthread_setaffinity_np check to the build system

Rather than trying to encode all of the rules in a header, lets just put
them in the build system where they belong. This fixes the build on
FreeBSD, which does have pthraed_setaffinity_np, but it's in a
pthread_np.h, not behind _GNU_SOURCE. FreeBSD also implements cpu_set
slightly differently, so additional changes would be required to get it
working right there anyway.

v2: - fix #define in autotools

Fixes: 9f1bbbdbbd
       ("util: try to fix the Android and MacOS build")
Cc: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/util/u_thread.h

@@ -36,10 +36,6 @@
 #include <signal.h>
 #endif
 
-#if defined(HAVE_PTHREAD) && !defined(ANDROID) && !defined(__APPLE__)
-#define HAVE_PTHREAD_SETAFFINITY
-#endif
-
 static inline thrd_t u_thread_create(int (*routine)(void *), void *param)
 {
    thrd_t thread;