remove old makefile targets
start enabling the use of parallel make

When building Macaculay2 may now build it with a command such as "make
-j4", with no ill effect.  The resulting parallelism has been disabled
in all but one of our recursive make processes, the building of the
FLINT library.  With the current scheme, we can start implementing
parallelism bit by bit.
Merge branch 'master' of github.com:Macaulay2/M2