Initial build with ninja generator fails with missing library

[Steven Bosnick]

The initial build of the project after using the Ninja CMake generator will fail with an error like:

        ninja: error: 'gtest/src/gtest-external-build/libgtest.a', needed by
        'src/acctcli/test/CommandLineTests', missing and no known rule to
         make it

This appears to be the effect of the CMake bug id 0014963 which was fixed in CMake version 3.2. We may be able to fix this bug for those using CMake version 3.2 and above by using the BUILD_BYPRODUCTS option of ExternalProject_Add().