Refactor libcxx makefile.  No functional changes intended.
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>


git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@145162 91177308-0d34-0410-b5e6-96231b3b80d8

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