Set CMAKE_SUPPRESS_REGENERATION in the Lib1 and Lib2 projects so that
their .vcxproj files do not contain references to ZERO_CHECK.  Such
references do not make sense when using the files in another .sln file.
This does not reduce the effectiveness of the test because real projects
that use include_external_msproject will have their own .vcxproj files not
generated by CMake anyway.