libobs/util: Make pthread_setname_np glibc only

Fixes an issue with certain distributions that don't use glibc
(particularly musl)

Closes jp9000/obs-studio#441

libobs/util/threading-posix.c

@@ -267,7 +267,7 @@ void os_set_thread_name(const char *name)
 	pthread_setname_np(name);
 #elif defined(__FreeBSD__)
 	pthread_set_name_np(pthread_self(), name);
-#elif !defined(__MINGW32__)
+#elif defined(__GLIBC__) && !defined(__MINGW32__)
 	pthread_setname_np(pthread_self(), name);
 #endif
 }