Clean up the usage of z_const and respect const usage within zlib.

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.

configure

@@ -70,6 +70,7 @@ shared=1
 solo=0
 cover=0
 zprefix=0
+zconst=0
 build64=0
 gcc=0
 old_cc="$CC"
@@ -96,7 +97,7 @@ do
 case "$1" in
     -h* | --help)
       echo 'usage:' | tee -a configure.log
-      echo '  configure [--zprefix] [--prefix=PREFIX]  [--eprefix=EXPREFIX]' | tee -a configure.log
+      echo '  configure [--const] [--zprefix] [--prefix=PREFIX]  [--eprefix=EXPREFIX]' | tee -a configure.log
       echo '    [--static] [--64] [--libdir=LIBDIR] [--sharedlibdir=LIBDIR]' | tee -a configure.log
       echo '    [--includedir=INCLUDEDIR] [--archs="-arch i386 -arch x86_64"]' | tee -a configure.log
         exit 0 ;;
@@ -119,6 +120,7 @@ case "$1" in
     -a*=* | --archs=*) ARCHS=`echo $1 | sed 's/.*=//'`; shift ;;
     --sysconfdir=*) echo "ignored option: --sysconfdir" | tee -a configure.log; shift ;;
     --localstatedir=*) echo "ignored option: --localstatedir" | tee -a configure.log; shift ;;
+    -c* | --const) zconst=1; shift ;;
     *)
       echo "unknown option: $1" | tee -a configure.log
       echo "$0 --help for help" | tee -a configure.log
@@ -171,7 +173,11 @@ if test "$gcc" -eq 1 && ($cc -c $test.c) >> configure.log 2>&1; then
     SFLAGS="${SFLAGS} -m64"
   fi
   if test "${ZLIBGCCWARN}" = "YES"; then
-    CFLAGS="${CFLAGS} -Wall -Wextra -pedantic"
+    if test "$zconst" -eq 1; then
+      CFLAGS="${CFLAGS} -Wall -Wextra -Wcast-qual -pedantic -DZLIB_CONST"
+    else
+      CFLAGS="${CFLAGS} -Wall -Wextra -pedantic"
+    fi
   fi
   if test -z "$uname"; then
     uname=`(uname -s || echo unknown) 2>/dev/null`