Allow specific extensions to be disabled by doing --enable-extensions=all,-ext_I_do_not_want

Bug #110900 r=pavlov

configure.in

@@ -2772,7 +2772,7 @@ dnl ========================================================
 dnl Need cookie and wallet to build mozilla, this should
 dnl not be the case, 18352 has been filed.
 dnl http://bugzilla.mozilla.org/show_bug.cgi?id=18352
-MOZ_EXTENSIONS_DEFAULT="cookie wallet content-packs xml-rpc xmlextras help transformiix venkman"
+MOZ_EXTENSIONS_DEFAULT=" cookie wallet content-packs xml-rpc xmlextras help transformiix venkman"
 MOZ_EXTENSIONS_ALL="$MOZ_EXTENSIONS_DEFAULT irc xmlterm inspector access-builtin ctl" 
 
 dnl Check if Thai Rendering Extension needs to be enabled
@@ -2793,6 +2793,9 @@ MOZ_ARG_ENABLE_STRING(extensions,
         MOZ_EXTENSIONS=""
     elif test "$option" = "default"; then
         MOZ_EXTENSIONS="$MOZ_EXTENSIONS $MOZ_EXTENSIONS_DEFAULT"
+    elif test `echo "$option" | grep -c ^-`; then
+        option=`echo $option | sed 's/^-//'`
+        MOZ_EXTENSIONS=`echo "$MOZ_EXTENSIONS" | sed "s/ ${option}//"`
     else
         MOZ_EXTENSIONS="$MOZ_EXTENSIONS $option"
     fi