--- configure.ac.old	2006-07-02 22:16:07.281399000 +0900
+++ configure.ac	2006-07-02 22:46:42.616440000 +0900
@@ -195,52 +195,59 @@
 
 dnl these are technically optional
 
-AC_MSG_CHECKING([for local zlib source])
 dnl zlib is needed for language level 3, and libpng
+AC_ARG_WITH(zlib, AC_HELP_STRING([--with-zlib],[include zlib support (local, system, no) (required for PNG and language level 3]))
 # we must define ZLIBDIR regardless because libpng.mak does a -I$(ZLIBDIR)
 # this seems a harmless default
 ZLIBDIR=src
-if test -d zlib; then
-	AC_MSG_RESULT([yes])
-	SHARE_ZLIB=0
-	ZLIBDIR=zlib
-else
-	AC_MSG_RESULT([no])
-	AC_CHECK_LIB(z, deflate, [
-	  AC_CHECK_HEADERS(zlib.h, [SHARE_ZLIB=1])
-	])
-fi
-if test -z "$SHARE_ZLIB"; then
-  AC_MSG_ERROR([I did not find a copy of zlib on your system.
-  Please either install it, or unpack a copy of the source in a
-  local directory named 'zlib'. See http://www.gzip.org/zlib/
-  for more information.
-  ])
+if test x$with_zlib != xno; then
+	AC_MSG_CHECKING([for local zlib source])
+	if test x$with_zlib != xsystem && test -d zlib; then
+		AC_MSG_RESULT([local])
+		SHARE_ZLIB=0
+		ZLIBDIR=zlib
+	else
+		AC_MSG_RESULT([system])
+		AC_CHECK_LIB(z, deflate, [
+		  AC_CHECK_HEADERS(zlib.h, [SHARE_ZLIB=1])
+		])
+	fi
+	if test -z "$SHARE_ZLIB"; then
+	  AC_MSG_ERROR([I did not find a copy of zlib on your system.
+	  Please either install it, or unpack a copy of the source in a
+	  local directory named 'zlib'. See http://www.gzip.org/zlib/
+	  for more information.
+	  ])
+	fi
 fi
 AC_SUBST(SHARE_ZLIB)
 AC_SUBST(ZLIBDIR)
 
 dnl png for the png output device; it also requires zlib
-LIBPNGDIR=src
-PNGDEVS=''
-PNGDEVS_ALL='$(DD)png48.dev $(DD)png16m.dev $(DD)pnggray.dev $(DD)pngmono.dev $(DD)png256.dev $(DD)png16.dev $(DD)pngalpha.dev'
-AC_MSG_CHECKING([for local png library source])
-if test -f libpng/pngread.c; then
-	AC_MSG_RESULT([yes])
-	SHARE_LIBPNG=0
-	LIBPNGDIR=libpng
-	PNGDEVS="$PNGDEVS_ALL"
-else
-	AC_MSG_RESULT([no])
-	AC_CHECK_LIB(png, png_check_sig, [
-	  AC_CHECK_HEADERS(png.h, [
-		SHARE_LIBPNG=1
+AC_ARG_WITH(png, AC_HELP_STRING([--with-png],[include PNG support (local, system, no)]))
+dnl set safe defaults
+	LIBPNGDIR=src
+	PNGDEVS=''
+if test x$with_png != xno; then
+	PNGDEVS_ALL='$(DD)png48.dev $(DD)png16m.dev $(DD)pnggray.dev $(DD)pngmono.dev $(DD)png256.dev $(DD)png16.dev $(DD)pngalpha.dev'
+	AC_MSG_CHECKING([for local png library source])
+	if test x$with_png != xsystem && test -f libpng/pngread.c; then
+		AC_MSG_RESULT([local])
+		SHARE_LIBPNG=0
+		LIBPNGDIR=libpng
 		PNGDEVS="$PNGDEVS_ALL"
-	  ], [SHARE_LIBPNG=0])
-	], [SHARE_LIBPNG=0], [-lz])
-fi
-if test -z "$PNGDEVS"; then
-  AC_MSG_NOTICE([disabling png output devices])
+	else
+		AC_MSG_RESULT([system])
+		AC_CHECK_LIB(png, png_check_sig, [
+		  AC_CHECK_HEADERS(png.h, [
+			SHARE_LIBPNG=1
+			PNGDEVS="$PNGDEVS_ALL"
+		  ], [SHARE_LIBPNG=0])
+		], [SHARE_LIBPNG=0], [-lz])
+	fi
+	if test -z "$PNGDEVS"; then
+	  AC_MSG_NOTICE([disabling png output devices])
+	fi
 fi
 AC_SUBST(SHARE_LIBPNG)
 AC_SUBST(LIBPNGDIR)