convert HAVE_GCC handling to modern GCC release numbering:
- HAVE_GCC=5 is now the default (vs. HAVE_GCC=53 we've been using for
GCC 5.4 and GCC 5.5.)
- remove some more GCC 4.8 code. we don't sup

convert HAVE_GCC handling to modern GCC release numbering:
- HAVE_GCC=5 is now the default (vs. HAVE_GCC=53 we've been using for
GCC 5.4 and GCC 5.5.)
- remove some more GCC 4.8 code. we don't support GCC 4 here.
- adjust set lists to gcc=5 from gcc=53.

add some basic HAVE_GCC=6 handling (totally unused so far.)

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limit to gcc, requested by joerg.

prevent gcc from combining memset+malloc -> calloc inside calloc so that
we end up calling ourselves recursively.

we have unistd.h

we want a lint library

merge changes, add build glue.