xwayland: Fix path in pkg-config with autoconf

When using autoconf/automake to build Xwayland, the actual path to
Xwayland is not fully qualified and refers to the "exec_prefix".

As a result, the path provided by the generated pkg-config file is wrong
when using autoconf to build the Xserver.

Fix the xwayland.pc file to also set the variable "prefix" and
"exec_prefix" so that the path to Xwayland is fully resolved.

Add those variables to the meson build as well for good measure.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
4 jobs for fix-xwayland-pkg-config-autoconf in 7 minutes and 30 seconds (queued for 40 minutes and 49 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #7769117

00:00:14

 
  Build And Test
passed autotools #7769118

00:07:16

passed meson #7769119

00:03:05

passed meson-noglamor #7769120

00:02:44