Makefile modifications for test target

On fresh clone of repo, "make test" used to give,
"make: *** No rule to make target `codec_unittest', needed by `test'.  Stop."
because gtest was not available. This message is not very helpful. Instead printing
that gtest is not available and the command to run to obtain gtest.
Merge pull request #144 from varunbpatil/better_make_test

Makefile modifications for test target