Linux needs -D_GNU_SOURCE for some functions, so enable it in Makefile

Makefile

@@ -16,6 +16,10 @@ CFLAGS += -I/usr/pkg/include
 LDFLAGS += -Wl,-rpath,/usr/local/lib -Wl,-rpath,/usr/pkg/lib
 endif
 
+ifeq ($(UNAME),Linux)
+CFLAGS += -D_GNU_SOURCE
+endif
+
 ifeq ($(DEBUG),1)
 # Extended debugging flags, macros shall be available in gcc
 CFLAGS += -gdwarf-2

src/layout.c

@@ -10,7 +10,6 @@
  * layout.c: Functions handling layout/drawing of window decorations
  *
  */
-#define _GNU_SOURCE
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>

src/mainx.c

@@ -8,7 +8,6 @@
  * See file LICENSE for license information.
  *
  */
-#define _GNU_SOURCE
 #include <stdio.h>
 #include <assert.h>
 #include <stdlib.h>