Added xmlterm as an optionally built extension, r=leaf.

Should not affect the default build, but xmlterm can now be built by
re-configuring with the '--with-extensions=xmlterm' option, on LINUX and
SOLARIS only. Do a "make" in the extensions/xmlterm directory and then
execute 'xmlterm' in the dist/bin directory to try it out.
(For some more fun: put mozilla/dist/bin in your PATH, cd to
dist/bin/chrome/navigator/skin/default, and try out the commands 'xls -i'
and 'xcat *.gif' inside the xmlterm!)

allmakefiles.sh

@@ -756,3 +756,17 @@ if [ `echo "$MOZ_EXTENSIONS" | grep -c pics` != 0 ]; then
   extensions/pics/tests/Makefile
 END_EXTENSIONS_PICS_MAKEFILES
 fi
+
+if [ `echo "$MOZ_EXTENSIONS" | grep -c xmlterm` != 0 ]; then
+  add_makefiles <<END_EXTENSIONS_XMLTERM_MAKEFILES
+  extensions/xmlterm/Makefile
+  extensions/xmlterm/base/Makefile
+  extensions/xmlterm/geckoterm/Makefile
+  extensions/xmlterm/linetest/Makefile
+  extensions/xmlterm/scripts/Makefile
+  extensions/xmlterm/tests/Makefile
+  extensions/xmlterm/ui/Makefile
+  extensions/xmlterm/ui/content/Makefile
+  extensions/xmlterm/ui/skin/Makefile
+END_EXTENSIONS_XMLTERM_MAKEFILES
+fi