gzflags() was put in gzwrite.c in order to be compiled exactly the
same as gzprintf(), so that it was guaranteed to return the correct
information. However that causes a static linkage to zlib to bring
in many routines that are often not used. All that is required to
duplicate the compilation environment of gzprintf() is to include
gzguts.h. So that is now done in zutil.c to assure that the correct
flags are returned.
A common request has been the ability to compile zlib to require no
other libraries. This --solo option provides that ability. The price
is that the gz*, compress*, and uncompress functions are eliminated,
and that the user must provide memory allocation and free routines to
deflate and inflate when initializing.
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.
