compiler.h: also check threads.h exists

In file included from ../src/log.h:8,
                 from ../src/backend/gl/glx.h:18,
                 from ../src/common.h:45,
                 from ../src/picom.c:32:
../src/compiler.h:98:11: fatal error: threads.h: No such file or directory
   98 | # include <threads.h>
      |           ^~~~~~~~~~~

src/compiler.h

@@ -94,7 +94,11 @@
 # define unreachable do {} while(0)
 #endif
 
-#ifndef __STDC_NO_THREADS__
+#ifndef __has_include
+# define __has_include(x) 0
+#endif
+
+#if !defined(__STDC_NO_THREADS__) && __has_include(<threads.h>)
 # include <threads.h>
 #elif __STDC_VERSION__ >= 201112L
 # define thread_local _Thread_local