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: https://llvm.org/svn/llvm-project/libcxx/trunk@145162 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed