Add --warn option to ./configure, instead of environment variable.

configure

@@ -85,6 +85,7 @@ zprefix=0
 zconst=0
 build64=0
 gcc=0
+warn=0
 old_cc="$CC"
 old_cflags="$CFLAGS"
 OBJC='$(OBJZ) $(OBJG)'
@@ -133,6 +134,7 @@ case "$1" in
     --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 ;;
+    -w* | --warn) warn=1; shift ;;
     *)
       echo "unknown option: $1" | tee -a configure.log
       echo "$0 --help for help" | tee -a configure.log
@@ -184,7 +186,7 @@ if test "$gcc" -eq 1 && ($cc -c $test.c) >> configure.log 2>&1; then
     CFLAGS="${CFLAGS} -m64"
     SFLAGS="${SFLAGS} -m64"
   fi
-  if test "${ZLIBGCCWARN}" = "YES"; then
+  if test "$warn" -eq 1; then
     if test "$zconst" -eq 1; then
       CFLAGS="${CFLAGS} -Wall -Wextra -Wcast-qual -pedantic -DZLIB_CONST"
     else