Use POSIX 2001 as the base standard.

Makefile

@@ -1,6 +1,7 @@
 .POSIX:
 
-REALCFLAGS =	${CFLAGS} -Wall -Iinclude -Iinclude/asm/gameboy -g -std=c99
+REALCFLAGS =	${CFLAGS} -Wall -Iinclude -Iinclude/asm/gameboy -g -std=c99 \
+		-D_POSIX_C_SOURCE=200112L
 
 # User-defined variables
 PREFIX =	/usr/local

src/asm/lexer.c

@@ -1,4 +1,3 @@
-#define _XOPEN_SOURCE 500
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

src/asm/main.c

@@ -5,7 +5,6 @@
  *
  */
 
-#define _XOPEN_SOURCE 500
 #include <math.h>
 #include <stdarg.h>
 #include <stdbool.h>

src/asm/symbol.c

@@ -5,7 +5,6 @@
  *
  */
 
-#define _XOPEN_SOURCE 500
 #include <assert.h>
 #include <stdio.h>
 #include <string.h>

src/asm/yaccprt1.y

@@ -1,5 +1,4 @@
 %{
-#define _XOPEN_SOURCE 500
 #include <ctype.h>
 #include <errno.h>
 #include <stdbool.h>

src/fix/main.c

@@ -14,7 +14,6 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#define _XOPEN_SOURCE 500
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>

src/link/main.c

@@ -1,4 +1,3 @@
-#define _XOPEN_SOURCE 500
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>