Backport XDT optional feature & binary check macros
This backports XDT_CHECK_OPTIONAL_FEATURE and XDT_CHECK_PACKAGE_BINARY to the 4.18.x series. We've started using these macros in non-core modules (like xfce4-terminal and xfce4-notifyd), as these make adding feature checks for X11 and Wayland (as well as needed binaries like wayland-scanner or glib-genmarshal) much easier and less error-prone. Unfortunately, this means that distro packagers cannot include these newer apps because there is no new stable release of xfce4-dev-tools.