my makefile
revert changes involving libgc tuning parameters

... GC_FREE_SPACE_DIVISOR and GC_INITIAL_HEAP_SIZE, because our examples and
tests are constrained by limits on both memory and time, and if we speed up a
run by consuming more memory, some of the examples will exceed their limits.
Merge branch 'master' of https://github.com/Macaulay2/M2