Google Test will be used for implementing unit tests. The first set of
unit tests will be for testing RpPng with various PNG image formats.
The Google Test code is currently identical to the release archive,
except for the removal of .travis.yml and appveyor.xml.
CMakeLists.txt: Added CTest support. (see cmake/gtest.cmake)
extlibs/CMakeLists.txt: Only build googletest libraries if testing is
enabled and some project depends on them.