improvement to config.w32

Please consider merging this tiny fix in order we can do the automatic PECL builds. Here's the build of the latest couchbase release using config.w32 patched

http://windows.php.net/downloads/pecl/releases/couchbase/1.1.5/

The problem with adding include/libcouchbase to includes is that it contains conflicting headers, so when a snapshot build is used, some extensions pull them in and fail to compile.