[cmake-developers] BUG: Pass --build-config option to ctest for multi-config ExternalProjects

[Taylor Braun-Jones]

For ExternalProjects built with a multi-config generator like "Visual
Studio 12" with `TEST_BEFORE_INSTALL ON` I need this patch:

https://github.com/Kitware/CMake/compare/master...nocnokneo:patch-1

Otherwise, the test step gives the error "Test not available without
configuration."

Thanks,
Taylor