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This ensures that if /usr/local/include has outdated header files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of master doesn't fail due to it picking up outdated header files.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: I9c4d7987adebb0780d3a0b08f27b666af128708d
Reviewed-on: http://review.couchbase.org/38361
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>