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.)

Newly compiled applications will use the gzgetc macro.

This patch adds the deflateResetKeep() function to retain the sliding
window for the next deflate operation, and fixes an inflateResetKeep()
problem that came from inflate() not updating the window when the
stream completed.  This enables constructing and decompressing a series
of concatenated deflate streams where each can depend on the history of
uncompressed data that precedes it.

This generalizes deflateSetDictionary() and inflateSetDictionary() to
permit setting the dictionary in the middle of a stream for raw deflate
and inflate.  This in combination with the Keep functions enables a
scheme for updating files block by block with the transmission of
compressed data, where blocks are sent with deflateResetKeep() to
retain history for better compression, and deflateSetDictionary() is
used for blocks already present at the receiver to skip compression but
insert that data in the history, again for better compression.  The
corresponding inflate calls are done on the receiver side.

Patch provided by Franz Schrober.

The Microsoft CAB file format compresses each block with completed
deflate streams that depend on the sliding window history of the
previous block in order to decode.  inflateResetKeep() does what
inflateReset() does, except the sliding window history from the
previous inflate operation is retained.

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.

Also added "-motley" to ZLIB_VERSION in zlib.h, so that versions
in-between 1.2.5.1 and 1.2.5.2 that are pulled down from github
can be identified as such if bugs are reported on them.