This patch allows zlib to compile cleanly with the -Wcast-qual gcc
warning enabled, but only if ZLIB_CONST is defined, which adds
const to next_in and msg in z_stream and in the in_func prototype.
A --const option is added to ./configure which adds -DZLIB_CONST
to the compile flags, and adds -Wcast-qual to the compile flags
when ZLIBGCCWARN is set in the environment.

During coverage testing it was discovered that these two lines could
never pull more bits, since the immediately preceding for loop assures
that all of the code's bits are already pulled.

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.