[cmake] Pass external project source directories to sub-configures

We're passing LLVM_EXTERNAL_PROJECTS to cross-compilation configures, so
we also need to pass the source directories of those projects, otherwise
configuration can fail from not finding them.

Differential Revision: https://reviews.llvm.org/D69076

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375157 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed