Addressing comments.

src/core/support/file_posix.c

@@ -33,7 +33,8 @@
 
 /* Posix code for gpr fdopen and mkstemp support. */
 
-#ifndef _POSIX_C_SOURCE
+#if !defined _POSIX_C_SOURCE || _POSIX_C_SOURCE < 200112L
+#undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200112L
 #endif