Make the default desktop shell client configurable

The default can be set by passing WESTON_SHELL_CLIENT as an argument
to configure, similarly to WESTON_NATIVE_BACKEND.

Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
diff --git a/src/shell.c b/src/shell.c
index 00e0f1c..ada60e8 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -587,7 +587,7 @@
 	section = weston_config_get_section(shell->compositor->config,
 					    "shell", NULL, NULL);
 	weston_config_section_get_string(section,
-					 "client", &s, LIBEXECDIR "/weston-desktop-shell");
+					 "client", &s, LIBEXECDIR "/" WESTON_SHELL_CLIENT);
 	shell->client = s;
 	weston_config_section_get_string(section,
 					 "binding-modifier", &s, "super");