build: silence warnings in in-tree build
build: silence warnings in in-tree build

Avoid the developer warnings from cmake when configuring libc++ as part of the
LLVM layout.  Setup the custom macro paths earlier to re-use the detection logic
prior to setting the project properties.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@260071 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx