add proper feature tests for putenv, getenv

src/main.c

@@ -14,7 +14,10 @@
  *   (at your option) any later version.                                   *
  *                                                                         *
  ***************************************************************************/
-
+#undef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
 #include <stdio.h>
 #include <unistd.h>
 #include <stdlib.h>