Define _REENTRANT prior to including math.h on macOS

Author: georgthegreat

libcxx/include/math.h

@@ -297,6 +297,10 @@ long double    truncl(long double x);
 #    pragma GCC system_header
 #  endif
 
+#  if defined(__APPLE__)
+#    define _REENTRANT
+#  endif
+
 #  if __has_include_next(<math.h>)
 #    include_next <math.h>
 #  endif