Forcing POSIX 1003.1-1990 removes locale_t from <locale.h>.
This breaks including libintl.h, since gettext is built with locale_t
in view.

Index: src/readkey.c
--- src/readkey.c.orig
+++ src/readkey.c
@@ -5,7 +5,7 @@
 /* readkey.c: read secret key phrase from terminal */
 
 #ifndef _POSIX_C_SOURCE
-#define _POSIX_C_SOURCE 1
+/* #define _POSIX_C_SOURCE 1 */
 #endif
 
 #ifdef HAVE_CONFIG_H
