Add configurable support for wl_shell and xdg-shell

If libwlmessage is built with the "--enable-xdg-shell"
configure flag AND the "WLMESSAGE_USE_XDG_SHELL"
environment variable is defined, then xdg-shell will be
used. Otherwise, fall back to wl_shell.

Change-Id: I65bd32315e0cadd1ebcf50464ffb1ea7eb3e053c
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>