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>

man/Makefile.am

@@ -7,6 +7,7 @@ endif
 MAN_SUBSTS = \
 	-e 's|__weston_native_backend__|$(WESTON_NATIVE_BACKEND)|g' \
 	-e 's|__weston_modules_dir__|$(pkglibdir)|g' \
+	-e 's|__weston_shell_client__|$(WESTON_SHELL_CLIENT)|g' \
 	-e 's|__version__|$(PACKAGE_VERSION)|g'
 
 SUFFIXES = .1 .5 .7 .man

man/weston.ini.man

@@ -141,7 +141,8 @@ The entries that can appear in this section are:
 .TP 7
 .BI "client=" file
 sets the path for the shell client to run. If not specified
-weston-desktop-shell is launched (string).
+.I __weston_shell_client__
+is launched (string).
 .TP 7
 .BI "background-image=" file
 sets the path for the background image file (string).