Improve const use in zlib-using code

If we define ZLIB_CONST before including zlib.h, zlib augments some
interfaces with const decorations.  By doing that we can keep our own
interfaces cleaner and can remove some unconstify calls.

ZLIB_CONST was introduced in zlib 1.2.5.2 (17 Dec 2011).  When
compiling with older zlib releases, you might now get compiler
warnings about discarding qualifiers.

CentOS 6 has zlib 1.2.3, but in 8e278b6, we removed support for the
OpenSSL release in CentOS 6, so it seems ok to de-support the zlib
release in CentOS 6 as well.

Reviewed-by: Tristan Partin <tristan@neon.tech>
Discussion: https://www.postgresql.org/message-id/flat/33462926-bb1e-7cc9-8d92-d86318e8ed1d%40eisentraut.org

Author: petere

contrib/pgcrypto/pgp-compress.c

@@ -113,7 +113,7 @@ compress_process(PushFilter *next, void *priv, const uint8 *data, int len)
 	/*
 	 * process data
 	 */
-	st->stream.next_in = unconstify(uint8 *, data);
+	st->stream.next_in = data;
 	st->stream.avail_in = len;
 	while (st->stream.avail_in > 0)
 	{

src/bin/pg_basebackup/bbstreamer_gzip.c

@@ -269,7 +269,7 @@ bbstreamer_gzip_decompressor_content(bbstreamer *streamer,
 	mystreamer = (bbstreamer_gzip_decompressor *) streamer;
 
 	zs = &mystreamer->zstream;
-	zs->next_in = (uint8 *) data;
+	zs->next_in = (const uint8 *) data;
 	zs->avail_in = len;
 
 	/* Process the current chunk */

src/bin/pg_basebackup/walmethods.c

@@ -705,7 +705,7 @@ typedef struct TarMethodData
 
 #ifdef HAVE_LIBZ
 static bool
-tar_write_compressed_data(TarMethodData *tar_data, void *buf, size_t count,
+tar_write_compressed_data(TarMethodData *tar_data, const void *buf, size_t count,
 						  bool flush)
 {
 	tar_data->zp->next_in = buf;
@@ -782,8 +782,7 @@ tar_write(Walfile *f, const void *buf, size_t count)
 #ifdef HAVE_LIBZ
 	else if (f->wwmethod->compression_algorithm == PG_COMPRESSION_GZIP)
 	{
-		if (!tar_write_compressed_data(tar_data, unconstify(void *, buf),
-									   count, false))
+		if (!tar_write_compressed_data(tar_data, buf, count, false))
 			return -1;
 		f->currpos += count;
 		return count;

src/include/c.h

@@ -75,6 +75,11 @@
 #include <libintl.h>
 #endif
 
+/* Define before including zlib.h to add const decorations to zlib API. */
+#ifdef HAVE_LIBZ
+#define ZLIB_CONST
+#endif
+
 
 /* ----------------------------------------------------------------
  *				Section 1: compiler characteristics