Define _POSIX_C_SOURCE to be 200112L, as a minimum for C99.

Strict platforms cannot be expected to accept C99 code as valid
when earlier standards versions are selected.

This helps the programs build on Solaris-like platforms (e.g.
illumos).

Fixes #3420

Signed-off-by: nia <nia@netbsd.org>
diff --git a/programs/aes/crypt_and_hash.c b/programs/aes/crypt_and_hash.c
index 1e03d43..5c70480 100644
--- a/programs/aes/crypt_and_hash.c
+++ b/programs/aes/crypt_and_hash.c
@@ -23,7 +23,7 @@
 /* Enable definition of fileno() even when compiling with -std=c99. Must be
  * set before config.h, which pulls in glibc's features.h indirectly.
  * Harmless on other platforms. */
-#define _POSIX_C_SOURCE 1
+#define _POSIX_C_SOURCE 200112L
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"