Fix Windows-cl.cmake so that at most one MSVC** variable is defined.

The expectation of users of the MSVC60, MSVC70, MSVC71, MSVC80, MSVC90
and the new MSVC10 variables is that at most one of them will be set
for any given build tree. This change enforces that expectation for
build trees using Makefile generators. It also fixes the one mismatch
in that expectation to be found in the Visual Studio generator world:
previously, the VS 7.1 generator would set *both* MSVC70 and MSVC71;
now, it only sets MSVC71.

With these changes, user expectations are now met, and the recently
introduced CheckCompilerRelatedVariables test should pass everywhere.

0 files changed, 0 insertions, 0 deletions