Since commit v3.5.0-rc1~32^2~1 (ExternalProject: Simplify `cmake
--build` configuration passing, 2016-01-19) we use the `$<CONFIG>`
generator expression to generate the `cmake --build . --config <config>`
value for the default BUILD_COMMAND instead of the CMAKE_CFG_INTDIR
placeholder value provided by multi-config generators.  However, some
projects have been abusing the old implementation detail by setting
CMAKE_CFG_INTDIR themselves to get a specific configuration.  Those
projects should be updated to set their own BUILD_COMMAND to get
non-default behavior.  Meanwhile we can be compatible with their
existing releases by detecting when CMAKE_CFG_INTDIR is not a
generator-provided placeholder and using its value instead.