Modified Makefile.common to handle compilation of projects inside and outside

Modified Makefile.common to handle compilation of projects inside and outside
of the llvm source directory.
The main modification was to add new environment variables: one set for llvm
entities and another set for source entities current being compiled.
This should make the Makefile more flexible and easier to understand as each
environment variable only does one thing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6679 91177308-0d34-0410-b5e6-96231b3b80d8