Added gtest and gmock under src/external/.

Added gtest and gmock under src/external/.

Source code copied from Google Test / Google Mock 1.6.0 (see
README.Gromacs for details).
Build system adapted to build tests against these versions of gtest and
gmock. Added advanced CMake variables GMX_USE_GTEST and GMX_USE_GMOCK,
which can be used to disable building the frameworks (and tests that use
them).

Google Test 1.6.0 handles test-thrown exceptions better (reports them as
failures instead of terminating the whole executable). With this
version, system-wide installation is no longer supported by Google, and
the officially supported way is to build the test framework as part of
the build system. Thus, the simplest way is to include the whole
framework in the source tree, which also makes it simpler to build the
tests.

Makes issue #639 obsolete (removes FindGMock.cmake completely) and
relates to #815 (removes FindCMake.cmake, which was copied from 2.8.2).

Change-Id: I021e81ea1529c8dea621c2d8b060c08987f9ca59