Move __has_builtin() fallback define to compiler.h

lib/common/compiler.h

@@ -196,6 +196,11 @@
     #define STATIC_BMI2 0
 #endif
 
+/* compat. with non-clang compilers */
+#ifndef __has_builtin
+#  define __has_builtin(x) 0
+#endif
+
 /* detects whether we are being compiled under msan */
 #if defined (__has_feature)
 #  if __has_feature(memory_sanitizer)