Update to llvm-config tool, by Erik Kidd:

Update to llvm-config tool, by Erik Kidd:

1. Check for Perl and only build llvm-config if its available.
2. Add some virtual components
3. Don't depend on "standard" location for Perl, but configured location
4. Document the tool with a POD file.

This version is now ready for testing by users.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27005 91177308-0d34-0410-b5e6-96231b3b80d8