Merge vestigial vsnprintf determination from zutil.h to gzguts.h.

Merge vestigial vsnprintf determination from zutil.h to gzguts.h.

This also moves some of the same from zconf.h to gzguts.h. A new
function, gzflags(), was created to pass the compilation flags
related to vsnprintf usage back to zlibCompileFlags() in zutil.c.
In the process, various compiler configuration files were updated
to include gzflags(), as well as the new gzgetc_() function added
when the gzgetc() macro was introduced in a previous patch.