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: https://llvm.org/svn/llvm-project/libcxx/trunk@260071 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed