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).

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1635>

meson.build

@@ -392,10 +392,16 @@ endif
 
 have_xwayland_initfd = false
 if have_wayland
+  xwayland_dep = dependency('xwayland', required: false)
+
   xwayland_path = get_option('xwayland_path')
   if xwayland_path == ''
+    if xwayland_dep.found()
+      xwayland_path = xwayland_dep.get_pkgconfig_variable('xwayland')
+    else
       xwayland_path = find_program('Xwayland').path()
     endif
+  endif
   cdata.set_quoted('XWAYLAND_PATH', xwayland_path)
 
   # For Xwayland authority file generation.
@@ -409,12 +415,18 @@ if have_wayland
 
   # For Xwayland -initfd usage
   use_initfd = get_option('xwayland_initfd')
+  if xwayland_dep.found()
+    xwayland_supports_initfd = xwayland_dep.get_pkgconfig_variable('have_initfd') == 'true'
+  else
     xwayland_options = run_command(xwayland_path, '-help')
+    xwayland_supports_initfd = xwayland_options.stderr().contains('-initfd')
+  endif
+
   if use_initfd.auto()
-    have_xwayland_initfd = xwayland_options.stderr().contains('-initfd')
+    have_xwayland_initfd = xwayland_supports_initfd
   else
     have_xwayland_initfd = use_initfd.enabled()
-    if have_xwayland_initfd and not xwayland_options.stderr().contains('-initfd')
+    if have_xwayland_initfd and not xwayland_supports_initfd
       error('XWayland -initfd support requested but not available')
     endif
   endif