build: Properly quote $enable_extensions

Otherwise test will complain if more then one extension in enabled, i.e.
when we have a space in the configure argument.

https://bugzilla.gnome.org/show_bug.cgi?id=676446

configure.ac

@@ -40,7 +40,7 @@ AC_ARG_ENABLE([extensions],
 	[],
 	[enable_extensions=$DEFAULT_EXTENSIONS]
 )
-if test x$enable_extensions = xall; then
+if test x"$enable_extensions" = xall; then
    enable_extensions="$ALL_EXTENSIONS"
 fi