Previously ./configure would use any output on stderr as an indication
that the compilation failed.  However if some compiler wrapper uses
stderr for some other purpose, e.g. distcc for nodes going down, then
./configure would not properly configure the build.  This problem was
noted by Mike Frysinger.  For backwards compatibility, ./configure
will revert to the old way, i.e. checking for anything on stderr, if
when it deliberately runs the compiler with an error, a zero exit
status is returned.

This allows deflate to generate the same output when continuing after
a Z_SYNC_FLUSH vs. using deflateSetDictionary() after a Z_FULL_FLUSH
or a deflateReset().  It also slightly improves compression when
flushing by providing two more strings to possibly match at the start
of the new block.

Previously, the bit buffer would hold 1 to 16 bits after "all" of the
output is provided after a Z_BLOCK deflate() call.  Now at most seven
bits remain in the output buffer after Z_BLOCK.  flush_pending() now
flushes the bit buffer before copying out the byte buffer, in order
for it to really flush as much as possible.

Z_PARTIAL_FLUSH would sometimes emit two empty static blocks instead
of one in order to provide enough lookahead for inflate to be able
to decode what was last compressed.  inflate no longer needs that
much lookahead, so this removes the possibility of emitting the
second empty static block.  Z_PARTIAL_FLUSH will now emit only one
empty static block.

This allows the insertion of multiple empty static blocks for the
purpose of efficiently bringing a stream to a byte boundary.

This avoids having to create useless variables for return values
that aren't needed.

Previously when doing an empty flush, a extra static or stored block
could be emitted before the requested empty static or stored block.
This patch prevents the emission of empty blocks by the deflate_*
functions.

The incorporation of the Z_BLOCK flush did not update the rejection
of lower ranked flushes immediately after higher ranked flushes with
no more input data.  This prevented an empty Z_SYNC_FLUSH right after
a Z_BLOCK flush, which would be desired to bring the deflate stream
to a byte boundary conditionally on whether or not it was already at
a byte boundary.  This patch re-ranks Z_BLOCK above Z_NO_FLUSH but
below Z_PARTIAL_FLUSH, allowing stronger empty flushes to follow a
Z_BLOCK flush.

Moves new function definitions in order to preserve shared library
compatibility with previous versions.  Also increases buffer size in
RPG binding and updates the documentation.

Newly compiled applications will use the gzgetc macro.

Also since gzread() will no longer return an error for an incomplete
gzip file, have gzclose() return an error if the last gzread() ended
in the middle of a gzip stream.

Z_BUF_ERROR was also being used for an unsuccessful gzungetc and for buffer
lengths that didn't fit in an int.  Those uses were changed to Z_DATA_ERROR
in order to assure that Z_BUF_ERROR occurs only when a premature end of
input occurs, indicating that gzclearerr() can be used.

Before, gzeof() would return true (accurately) when the last read request
went just up to the end of the uncompressed data.  In the analogous case,
feof() would return false, only returning true when a read request goes
past the end of the file.  This patch corrects gzeof() to behave in the
same way as feof(), as noted in the zlib.h documentation.

inflate() avoided that allocation normally, until it was modified to
update the window on a normal completion so that inflateResetKeep()
could work.  This patch restores that behavior, but only when
Z_FINISH is used successfully to complete an inflation of a stream in
a single call of inflate().  The comments in zlib.h have been updated
accordingly.

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 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.

Add a cover target in Makefile and the test/infcover.c test program
to cover all of the code lines in the inf*.c source files.  The
coverage is run with memory allocation checking in order to expose
memory leaks.  The coverage testing is run using:

    ./configure --cover && make cover

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.

Due to earlier changes in the error checking in inflate_table(), the
code to fill in a table for an incomplete code handled cases that can
never actually occur.  This simplifies that code to handle the only
possible case, which is a single empty table entry for a code with
a single symbol with a length of one bit.