Add --without-mac-twain to configure

INSTALL

@@ -204,6 +204,9 @@ These are:
   --without-hal.  If you want to build the Linux Input controller module
      without HAL support, you can use the --without-hal option.
 
+  --without-mac-twain.  If you don't want to compile the Mac OS X
+    TWAIN plug-in, you can use the --without-mac-twain option.
+
   --with-gif-compression=[lzw|rle|none].  Allows to tune the compression
      algorithm used by the GIF plug-in. If you are afraid of Unisys' LZW
      patent (which should have expired in most countries by now), you

configure.ac

@@ -1745,13 +1745,18 @@ esac
 # Check for Mac OS X TWAIN framework
 ####################################
 
+AC_ARG_WITH(mac-twain,  [  --without-mac-twain     don't build the Mac TWAIN plug-in])
+
 mac_twain_ok=no
-AC_MSG_CHECKING([for Mac OS X TWAIN support])
-AC_TRY_CPP([
+if test "x$with_mac_twain" != xno; then
+  AC_MSG_CHECKING([for Mac OS X TWAIN support])
+  AC_TRY_CPP([
 #include <Carbon/Carbon.h>
 #include <TWAIN/TWAIN.h>
-], mac_twain_ok=yes)
-AC_MSG_RESULT($mac_twain_ok)
+  ], mac_twain_ok=yes)
+  AC_MSG_RESULT($mac_twain_ok)
+fi
+
 AM_CONDITIONAL(HAVE_MAC_TWAIN, test "x$mac_twain_ok" = xyes)