build: Use Xwayland pkg-config if available
Xwayland in its main development branch now provides a pkg-config file which can be used to determine Xwayland installation path and various options enabled at build time.
Change mutter build system to optionally use that mechanism if available. If not (as with Xwayland from the current stable branch), keep the fallback mechanisms (basically search for the Xwayland and parse its command line options to determine if initfd is available).