Change makefile comment to refer to libc++ instead of libcpp.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@180135 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid bash specific functionality to work with any POSIX shell
implementing $(( )).


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@180139 91177308-0d34-0410-b5e6-96231b3b80d8