Adjust Makefile for C warnings and compiler optimizations

Separate Warnings into separate variable and set optimization flags in
one location
Remove #if 0 code from assemble_fill_ext_v which was changed to fix warnings
Remove #if 0 code from current_elem_overlaps_interface that was modified
because of compiler warnings
Remove #if constant from mtc_chilton_coburn that were modified due to compiler warnings
Remove #if 0 code from shear_to_shell that was causing previous warnings
Merge pull request #26 from wortiz/makefile-adjust

Adjust Makefile for C warnings and compiler optimizations
Merge pull request #27 from wortiz/fix-if-constant

Remove #if CONSTANT code that isn't being used