Have CMake error out on Windows when user wants OMPT support

Currently, OMPT support requires the weak attribute which isn't supported
on Windows.  This patch has CMake error out when LIBOMP_OMPT_SUPPORT=true
and the users is building on Windows.
http://lists.cs.uiuc.edu/pipermail/openmp-dev/2015-June/000692.html

Patch by Jonas Hahnfeld

llvm-svn: 239912