Don't define __ISO_C_VISIBLE unconditionally.

EdSchouten · EdSchouten · commit f9fd21c96f3a · 2015-01-07T22:20:56.000+01:00
__ISO_C_VISIBLE is already defined on FreeBSD. By default, it has a
value of 2011. This causes a lot of compiler warnings.
diff --git a/src/openlibm.h b/src/openlibm.h
@@ -57,7 +57,9 @@ extern const union __nan_un {
 //VBS begin
 #define __MATH_BUILTIN_CONSTANTS
 #define	__MATH_BUILTIN_RELOPS
+#ifndef __ISO_C_VISIBLE
 #define __ISO_C_VISIBLE 1999
+#endif
 //VBS end
 
 #ifdef __MATH_BUILTIN_CONSTANTS
