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