Addition of GTEST_INCLUDE_DIR requires dependencies

A change to lp:unity in rev 2884 is causing build failures in ppa:unity-team/staging:
https://launchpadlibrarian.net/122383545/buildlog_ubuntu-quantal-amd64.unity_6.10.0bzr2884pkg0quantal0_FAILEDTOBUILD.txt.gz

or a backup is here:
http://paste.ubuntu.com/1343782/

and also attached.

The change to CMakeLists.txt appears to enable variables that are only valid when libgtest-dev and google-mock are installed.

[Test Case]
 - build in a pure pbuilder chroot with no additional depenencies

Note: this made it through the auto-merger because libgtest-dev and google-mock (and others) are added to the pbuilder prior to executing the build. This is needed to execute the unit tests. However, in launchpad, this is not done and therefore the build fails.