Refactor libcxx makefile.  No functional changes intended.

Besides cleaning up the repetition in the installhdrs target, the point of this
change is to provide a separate do-installhdrs target that can be used directly
from clang's runtime/libcxx makefile to install a copy of the headers along
with clang.  <rdar://problem/10397739>

llvm-svn: 145162
1 file changed
tree: 68f9ce934643da661538ca5f563858412a735de1
  1. clang/
  2. compiler-rt/
  3. debuginfo-tests/
  4. libcxx/
  5. libcxxabi/
  6. lldb/
  7. llvm/
  8. polly/