VS: Fix VS >= 10 build tool selection from CMAKE_MAKE_PROGRAM

When a build tree contains an explicit CMAKE_MAKE_PROGRAM cache entry, the
cmGlobalVisualStudio10Generator::GenerateBuildCommand method must
recognize when it is devenv and defer to cmGlobalVisualStudio7Generator.
Similarly, cmGlobalVisualStudio7Generator::GenerateBuildCommand must
ignore any suggestion to use msbuild and force use of devenv.
Merge topic 'cleanup-build-commands' into next

2188d96 VS: Fix VS >= 10 build tool selection from CMAKE_MAKE_PROGRAM