[CMake] Disallow direct configuration

As a first step, this allows us to generalize the detection of
standalone builds and make it fully compatible when building in
llvm/runtimes/ which automatically sets OPENMP_STANDLONE_BUILD.

Differential Revision: https://reviews.llvm.org/D40080

llvm-svn: 319341
GitOrigin-RevId: 3e921d3c52542b3284c4bd0ef29a70511d38c79c
10 files changed