implement strcasestr since mingw doesn't have it

main.h

@@ -1,8 +1,6 @@
 #ifndef MAIN_H
 #define MAIN_H
 
-#define _GNU_SOURCE
-
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdint.h>
@@ -11,6 +9,7 @@
 #include <string.h>
 #include <math.h>
 #include <limits.h>
+#include <ctype.h>
 
 #include <tox/tox.h>
 #include <tox/toxav.h>

util.c

@@ -39,6 +39,26 @@ void* file_raw(char *path, uint32_t *size)
     return data;
 }
 
+_Bool strcasestr(const char *a, const char *b)
+{
+    const char *c = b;
+    while(*a) {
+        if(tolower(*a) != tolower(*c)) {
+            c = b;
+        }
+
+        if(tolower(*a) == tolower(*c)) {
+            c++;
+            if(!*c) {
+                return 1;
+            }
+        }
+        a++;
+    }
+
+    return 0;
+}
+
 static void to_hex(char_t *a, char_t *p, int size)
 {
     char_t b, c, *end = p + size;

util.h

@@ -6,6 +6,9 @@
  */
 void* file_raw(char *path, uint32_t *size);
 
+/* returns non-zero if substring is found */
+_Bool strcasestr(const char *a, const char *b);
+
 /* convert tox id to string
  *  notes: dest must be (TOX_FRIEND_ADDRESS_SIZE * 2) bytes large, src must be TOX_FRIEND_ADDRESS_SIZE bytes large
  */