update Makefile.orig and add instructions to rebuild fpoptimizer.cc
Use the LIBMESH_DIR environment variable, if it exists, to find a
valid libmesh installation.
Miscellaneous fixes to new README file:

* Fix typo.
* Use 'rm -f' for .dep and fpoptimizer.cc
* List known limitations on the process of rebuilding fpoptimizer.cc
Merge branch 'fpoptimizer'

Branch from Daniel Schwen with a few improvements for finding the
appropriate LIBMESH_DIR.