Also correct typo for 64-bit debug build.

Without this fix, it would be possible to construct inputs to puff
that would cause it to segfault.

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.

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.

This makes build-testing and installing the minizip/miniunzip programs
as simple as "autoreconf -if && ./configure --enable-demos && make &&
make install".  Without --enable-demos, the makefile will only build
and install the library, as before.  Helped by Mike Frysinger.

minizip/miniunzip were not intended to be general-purpose installed
utilities, but they can be useful from time to time as a lightweight
substitute for zip/unzip.  You can also use them to quickly test that
the library installation procedure worked.

Instead of using relative paths directly, use paths relative to
top_srcdir and top_builddir to refer to source files and built files,
respectively.

Note that the toplevel zlib configure script still does not have any
special support for out-of-tree builds.  But now you can do

	(cd contrib/minizip && autoreconf -fis)

	mkdir -p BUILD/test
	cp *.c *.h *.in zlib.map configure zlib.3 BUILD
	cp test/*.c BUILD/test
	(cd BUILD && ./configure --shared)
	(cd BUILD && make)

	mkdir -p BUILD/contrib/minizip
	cd BUILD/contrib/minizip
	../../../contrib/minizip/configure
	make

While at it, move the include path and library path settings to
CPPFLAGS and LDFLAGS respectively instead of setting both in CFLAGS.
Thanks to Mike Frysinger for advice.

Trying to build the minizip utility from contrib/minizip after an
autoreconf -f:

	libtool: link: gcc -g -O2 -o minizip minizip.o
	minizip.o: In function `getFileCrc':
	/tmp/zlib/contrib/minizip/minizip.c:211: undefined reference to `crc32'
	minizip.o: In function `main':
	/tmp/zlib/contrib/minizip/minizip.c:378: undefined reference to `zipOpen64'
	/tmp/zlib/contrib/minizip/minizip.c:451: undefined reference to `zipOpenNewFileInZip3_64'
	/tmp/zlib/contrib/minizip/minizip.c:502: undefined reference to `zipCloseFileInZip'
	/tmp/zlib/contrib/minizip/minizip.c:509: undefined reference to `zipClose'
	/tmp/zlib/contrib/minizip/minizip.c:485: undefined reference to `zipWriteInFileInZip'
	collect2: error: ld returned 1 exit status

The cause: contrib/minizip/Makefile.am does not specify that minizip
needs to be linked to libminizip.  With some linkers (e.g., GNU
binutils without --copy-dt-needed-entries), an indirect dependency
cannot be used to resolve symbols, so link to libz for crc32(), too.

Trying to build miniunzip utility from contrib/minizip after an
autoreconf -f produces

	[...]
	In file included from minizip.c:61:0:
	zip.h:50:18: fatal error: zlib.h: No such file or directory

unless zlib is already installed.  Use AM_CFLAGS to set the include
path and library path to point to the just-build copy of zlib to
fix this.  (This was already done for libminizip but not the binaries
that use it before this patch.)