Use gmock/gtest headers when PRIVATELIB is defined

The move to /usr/include/private prefixed paths seems to require a bit more
effort in order to compile programs.

Install the headers to /usr/include/private/g{mock,test}/... and automatically
include /usr/include/private in GTESTS_CXXFLAGS to make compilation seamless. I
will work on the more global problem later with @bdrewery.