Added a section on all of the wonderful Makefile variables used to locate

Added a section on all of the wonderful Makefile variables used to locate
LLVM.
Added information on how to use the autoconf code to specify LLVM's
location and setup separate source and object trees.
Fixed some of the wording to be more accurate, especially regarding the
building of libraries.
Added information on where built files are placed.
Added a link to the llvmdev mailing list so that they can ask us questions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9173 91177308-0d34-0410-b5e6-96231b3b80d8