Drop the VS >= 7 generator's global Configurations member and instead
lookup configurations using cmMakefile::GetConfigurations where needed.
This avoids accumulating all CMAKE_CONFIGURATION_TYPES values ever
encountered by a project() or enable_language() command and allows
the final value to be used in each directory.  We don't officially
support per-directory CMAKE_CONFIGURATION_TYPES values but we certainly
should not generate configurations not in the final value in the top
level directory.