Generalize paths to source code in build.py and buildlib.py, to use
    config_files.xml.
    Update Makefile to be able to handle different locations of of mct
    and pio. Also add an if statement place holder for furture CESM
    share repository.
    Update case_clone.py to be able to handle the future CESM share
    repository.
    Add get_src_root to utils.py, similar to get_cime_root. Needed this
    to correct an assumption in utilities.py that srcroot=cimeroot/...
    Added place holder if statement in scripts_regression_tests for the
    new CESM share repository.
    Did some minor cleanup of build_scripts.

There's a couple of place holder if statements that I added that will be
updated when we bring in the new CESM share repository.

Build_scripts and CMake files will still need to be updated when the
CESM share repository is ready. I didn't include any changes since
the structure of the CESM share repository will be changing.

Test suite: scripts_regression_tests
Test baseline:
Test namelist changes:
Test status: bit for bit

Fixes [CIME Github issue #]

User interface changes?:

Update gh-pages html (Y/N)?:

Code review: