Remove all implicit definitions of compiler provided functions.

<strings.h> cause strncasecmp to be define.

src/asm/lexer.c

@@ -1,6 +1,8 @@
+#define _XOPEN_SOURCE 500
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <ctype.h>
 
 #include "asm/asm.h"

src/asm/symbol.c

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

src/asm/yaccprt1.y

@@ -1,10 +1,12 @@
 %{
+#define _XOPEN_SOURCE 500
 #include <ctype.h>
 #include <errno.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 
 #include "asm/symbol.h"
 #include "asm/asm.h"