Fixing Makefile to avoid unnecessary recompilation of *all* checkers
each time any of the .cpp files under ASaP is modified.
Removed an erroneous assertion (and added explanation)
Added case in isSubtypeOf where the LHS may not appear to be a suptype of the RHS.
This may happen when an unsupported implicit cast from function to function pointer
causes type to be incompatible at the assignment site, resulting in an ASaP assignment error.