xbps-repo(8): 'search' target is now case insensitive.

Along with this change, compat code has been added (from NetBSD)
for systems that don't have it. The compat code has been reorganized
to be in a common place and its prototypes in compat.h.

The configure scripts checks if strcasestr() is available, and
uses compat code if not found.

This fixes issue #2 on github.com/vanilla/xbps.

lib/compat/strlcat.c

@@ -19,7 +19,7 @@
 #include <sys/types.h>
 #include <string.h>
 
-#include "strlcat.h"
+#include "compat.h"
 
 /*
  * Appends src to string dst of size siz (unlike strncat, siz is the