Avoid including system gtest header of an incompatible version

With a gtest 1.7 on my system, the `make check` command reports a ton of undefined reference of `EqFailure`s. Obviously, the message types were changed from some internal string data structure to `std::string`. The current Makefile includes the system gtest headers, but does not link in its libraries.

Given that `make check` fails when the gtest directory is not there, including files under `$(srcdir)/gtest/include` doesn't break more things, and should be enough.