autotools: use pkg-config instead of AC_PATH_XTRA
Hello !
Building dbus statically with autotools on nixpkgs doesn't work because X11 libraries aren't found. Even though this could be a nixpkgs' issue, it seems like AC_PATH_XTRA isn't well documented and is obsolete, pkg-config is a more standard way of handling dependencies and it seems to fix the build.
We could maintain this patch on our side but patching is quite inconvenient and has proven to be a significant technical debt when used extensively.