configure.ac: make use of wayland-scanner.pc

configure.ac: make use of wayland-scanner.pc

Currently we use the wayland-scanner executable as found with
AC_PATH_PROG, and then check the presence of wayland-scanner.pc

Currently the latter is unused even if AC_PATH_PROG fails to find the
binary. Rework things to use the pkg-config variable as a fall-back.

Cc: Andrew Oakley <aoakley@espial.com>
Cc: Bill Spitzak <spitzak@gmail.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Andrew Oakley <aoakley@espial.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Quentin Glidic <sardemff7+wayland@sardemff7.net>