diff --git a/Makefile b/Makefile
index 4910113a..b2ba8126 100644
--- a/Makefile
+++ b/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
diff --git a/src/asm/lexer.c b/src/asm/lexer.c
index 868f50a5..d212ef12 100644
--- a/src/asm/lexer.c
+++ b/src/asm/lexer.c
@@ -1,4 +1,3 @@
-#define _XOPEN_SOURCE 500
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
diff --git a/src/asm/main.c b/src/asm/main.c
index 46ff501d..e6232b03 100644
--- a/src/asm/main.c
+++ b/src/asm/main.c
@@ -5,7 +5,6 @@
  *
  */
 
-#define _XOPEN_SOURCE 500
 #include <math.h>
 #include <stdarg.h>
 #include <stdbool.h>
diff --git a/src/asm/symbol.c b/src/asm/symbol.c
index 92adcffa..34f08445 100644
--- a/src/asm/symbol.c
+++ b/src/asm/symbol.c
@@ -5,7 +5,6 @@
  *
  */
 
-#define _XOPEN_SOURCE 500
 #include <assert.h>
 #include <stdio.h>
 #include <string.h>
diff --git a/src/asm/yaccprt1.y b/src/asm/yaccprt1.y
index 5f5c4842..0f10c470 100644
--- a/src/asm/yaccprt1.y
+++ b/src/asm/yaccprt1.y
@@ -1,5 +1,4 @@
 %{
-#define _XOPEN_SOURCE 500
 #include <ctype.h>
 #include <errno.h>
 #include <stdbool.h>
diff --git a/src/fix/main.c b/src/fix/main.c
index f43f96ba..d7d0564b 100644
--- a/src/fix/main.c
+++ b/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>
diff --git a/src/link/main.c b/src/link/main.c
index 2507bb56..b244d731 100644
--- a/src/link/main.c
+++ b/src/link/main.c
@@ -1,4 +1,3 @@
-#define _XOPEN_SOURCE 500
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>