Define macros to fix implicit function declaration errors.
---
--- a/src/expr.c
+++ b/src/expr.c
@@ -13,6 +13,8 @@
  * Parses and evaluates expressions.
  ********************************************************************/
 
+#define _GNU_SOURCE
+
 #include "tfconfig.h"
 #include <math.h>
 #include <limits.h>
--- a/src/history.c
+++ b/src/history.c
@@ -13,6 +13,8 @@
  * Handles text queuing and file I/O for logs.                  *
  ****************************************************************/
 
+#define _GNU_SOURCE
+
 #include <limits.h>
 #include "tfconfig.h"
 #include "port.h"
--- a/src/pattern.c
+++ b/src/pattern.c
@@ -10,6 +10,8 @@
  * Regexp wrappers and glob pattern matching.
  */
 
+#define _GNU_SOURCE
+
 #include "tfconfig.h"
 #if HAVE_LOCALE_H
 # include <locale.h>
--- a/src/port.h
+++ b/src/port.h
@@ -6,6 +6,8 @@
  *  General Public License.  See the file "COPYING" for details.
  ************************************************************************/
 
+#define _GNU_SOURCE
+
 #ifndef PORT_H
 #define PORT_H
 
@@ -86,6 +88,7 @@
 #if STDC_HEADERS
 # include <stdlib.h>
 # include <string.h>
+# include <strings.h> /* strcasecmp() */
 #else
 extern void free();
 # if HAVE_MEMORY_H
--- a/src/signals.c
+++ b/src/signals.c
@@ -8,6 +8,8 @@
 
 /* Signal handling, core dumps, job control, and interactive shells */
 
+#define _POSIX_C_SOURCE 199309L /* for siginfo_t */
+
 #include "tfconfig.h"
 #include <signal.h>
 #include <setjmp.h>
--- a/src/socket.c
+++ b/src/socket.c
@@ -16,6 +16,8 @@
  * Autologin handled here.
  ***************************************************************/
 
+#define _POSIX_C_SOURCE 199309L /* for siginfo_t */
+
 #include "tfconfig.h"
 #include <sys/types.h>
 #if HAVE_SYS_SELECT_H
--- a/src/util.c
+++ b/src/util.c
@@ -15,6 +15,8 @@
  * Mail checker
  */
 
+#define _GNU_SOURCE
+
 #include "tfconfig.h"
 #if HAVE_LOCALE_H
 # include <locale.h>
--- a/src/variable.c
+++ b/src/variable.c
@@ -10,6 +10,8 @@
  * Internal and environment variables *
  **************************************/
 
+#define _GNU_SOURCE /* timercmp() */
+
 #include "tfconfig.h"
 #include "port.h"
 #include "tf.h"