Add an undocumented CMake_TEST_EXTERNAL_CMAKE option to name an external
CMake 'bin' directory.  Skip all main CMake binary builds and instead
configure the Tests directory to run using the external CMake provided.
This will provide a means to exercise the CMake test suite generating
for target platforms and compilers with which the CMake source does not
build.  That will allow us to raise the level of C++ features required
of a compiler to build our source while retaining tests for generating
projects with older compiler tools.