Make LINK_POLLY_INTO_TOOLS work with the CMake build

The option check was being performed after config.h/llvm-config.h substitution,
generating incorrect macro definitions.

Fixes PR19614.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210311 91177308-0d34-0410-b5e6-96231b3b80d8