Make zlib API constness-aware, take 2.

ZLIB_CONST must be defined before including zlib.h to expose z_const,
*AND* z_const shouldn't be used unless it's defined, because older
versions of zlib don't define it even if you define ZLIB_CONST.

While we're at it, throw in some DIAG_OFF(cast-qual)/DIAG_ON(cast-qual)
pairs to suppress unavoidable "cast throws away const qualification"
warnings.

The original "make zlib constness-aware" change also removed an
unnecessary include of <zlib.h> from wiretap/wtap.c, so we do that as
well.

Change-Id: I3c5269a8fbc54bbbb4d316544cc7b8fa30614c19
Reviewed-on: https://code.wireshark.org/review/12675
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>

epan/dissectors/packet-spdy.c

@@ -42,6 +42,7 @@
 #include "packet-ssl.h"
 
 #ifdef HAVE_LIBZ
+#define ZLIB_CONST
 #include <zlib.h>
 #endif
 
@@ -961,7 +962,13 @@ static guint8* spdy_decompress_header_block(tvbuff_t *tvb,
   const guint8 *hptr = tvb_get_ptr(tvb, offset, length);
   guint8 *uncomp_block = (guint8 *)wmem_alloc(wmem_packet_scope(), DECOMPRESS_BUFSIZE);
 
+#ifdef z_const
+  decomp->next_in = (z_const Bytef *)hptr;
+#else
+DIAG_OFF(cast-qual)
   decomp->next_in = (Bytef *)hptr;
+DIAG_ON(cast-qual)
+#endif
   decomp->avail_in = length;
   decomp->next_out = uncomp_block;
   decomp->avail_out = DECOMPRESS_BUFSIZE;

epan/tvbuff_zlib.c

@@ -28,6 +28,7 @@
 #include <string.h>
 
 #ifdef HAVE_LIBZ
+#define ZLIB_CONST
 #include <zlib.h>
 #endif
 

wiretap/file_wrappers.c

@@ -45,9 +45,11 @@
 #include <string.h>
 #include "wtap-int.h"
 #include "file_wrappers.h"
+#include <wsutil/ws_diag_control.h>
 #include <wsutil/file_util.h>
 
 #ifdef HAVE_LIBZ
+#define ZLIB_CONST
 #include <zlib.h>
 #endif /* HAVE_LIBZ */
 
@@ -473,7 +475,13 @@ zlib_read(FILE_T state, unsigned char *buf, unsigned int count)
         ret = inflate(strm, Z_NO_FLUSH);
 #endif
         state->avail_in = strm->avail_in;
+#ifdef z_const
+DIAG_OFF(cast-qual)
+        state->next_in = (unsigned char *)strm->next_in;
+DIAG_ON(cast-qual)
+#else
         state->next_in = strm->next_in;
+#endif
         if (ret == Z_STREAM_ERROR) {
             state->err = WTAP_ERR_DECOMPRESS;
             state->err_info = strm->msg;
@@ -1672,7 +1680,13 @@ gzwfile_write(GZWFILE_T state, const void *buf, guint len)
             n = state->size - strm->avail_in;
             if (n > len)
                 n = len;
+#ifdef z_const
+DIAG_OFF(cast-qual)
+            memcpy((Bytef *)strm->next_in + strm->avail_in, buf, n);
+DIAG_ON(cast-qual)
+#else
             memcpy(strm->next_in + strm->avail_in, buf, n);
+#endif
             strm->avail_in += n;
             state->pos += n;
             buf = (const char *)buf + n;
@@ -1688,10 +1702,12 @@ gzwfile_write(GZWFILE_T state, const void *buf, guint len)
 
         /* directly compress user buffer to file */
         strm->avail_in = len;
-#if ZLIB_CONST
+#ifdef z_const
         strm->next_in = (z_const Bytef *)buf;
 #else
+DIAG_OFF(cast-qual)
         strm->next_in = (Bytef *)buf;
+DIAG_ON(cast-qual)
 #endif
         state->pos += len;
         if (gz_comp(state, Z_NO_FLUSH) == -1)

wiretap/wtap.c

@@ -27,10 +27,6 @@
 #include <sys/types.h>
 #endif
 
-#ifdef HAVE_LIBZ
-#include <zlib.h>
-#endif
-
 #include "wtap-int.h"
 
 #include "file_wrappers.h"