fix up files for mfshell so that strdup is available on old systems

mfshell/main.c

@@ -18,6 +18,7 @@
  */
 
 #define _POSIX_C_SOURCE 200809L // for strdup
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <openssl/ssl.h>
 #include <stddef.h>

mfshell/mfshell.c

@@ -23,6 +23,7 @@
 #else
 #define _WITH_GETLINE           // on freebsd for getline
 #endif
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <stdio.h>
 #include <stdlib.h>

mfshell/options.c

@@ -17,6 +17,7 @@
  */
 
 #define _POSIX_C_SOURCE 200809L // for strdup
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <stdio.h>
 #include <getopt.h>