[CMake][runtimes] Support monorepo layout with runtimes build

We introduce a new variable LLVM_ENABLE_RUNTIMES which works
similarly to LLVM_ENABLE_PROJECTS and allows specifying runtimes
that will be enabled in the runtimes build.

Differential Revision: https://reviews.llvm.org/D40233

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319107 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed