CMake: Search project include dirs before /usr/local/include

This ensures that if /usr/local/include has outdated header files
(e.g. you previously compiled and installed CB 2.5.1), then the build
of master doesn't fail due to it picking up outdated header files.

Change-Id: I040a507cbfb97dbf0c7bdcd4210c5d18870949ae
Reviewed-on: http://review.couchbase.org/38360
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>