Restore support for MacOS < 10.15

[distler]

Commit ccc753d removed support for MacOS < 10.15 because of the use of aligned_alloc. This could be worked around by

#if PRE_HIGH_SIERRA
	posix_memalign((void **) &buf, alignof(struct mbuf_block), size);
#else
	buf = (char *) aligned_alloc(alignof(struct mbuf_block), size);
#endif

in src/cxx_supportlib/MemoryKit/mbuf.cpp (two instances) and reverting some of the "code-cleanup" in Commit a067758.