replace stub with working strcasestr

src/string/strcasestr.c

+#define _GNU_SOURCE
 #include <string.h>
 
 char *strcasestr(const char *h, const char *n)
 {
-	//FIXME!
-	return strstr(h, n);
+	size_t l = strlen(n);
+	for (; *h; h++) if (!strncasecmp(h, n, l)) return (char *)h;
+	return 0;
 }