Added installheaders target.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@148399 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile b/Makefile
index 316cf8e..9546857 100644
--- a/Makefile
+++ b/Makefile
@@ -30,6 +30,8 @@
 
 clean::
 
+installheaders:: do-installhdrs
+
 # The do-installhdrs target is also used by clang's runtime/libcxx makefile.
 do-installhdrs:
 	mkdir -p $(HEADER_DIR)/c++/v1/ext