gzwrite.c had hard-coded parameters to deflateInit2() which could
contradict compile-time options for the use of less memory and fewer
code bits. This patch suggested by Karsten Saunte fixes that.
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.
