Avoid name clashes, set _GTEST_AND_GMOCK_INSIDE_FINDING_PROJECT on found.

This allows us to avoid having GTEST_LIBRARY_LOCATION etc be used as
the find path on the next configure, which may result in gmock not
actually being built
Merge pull request #21 from smspillaz/gmock-cmake.item_20

Avoid name clashes, set _GTEST_AND_GMOCK_INSIDE_FINDING_PROJECT on found.