build: make default backend configurable

Instead of hardcoding drm-backend.so as the default if environment
presents neither Wayland nor X11, have a ./configure option to change
it. It still defaults to drm-backend.so, if not given.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

man/Makefile.am

@@ -1,6 +1,7 @@
 man_MANS = weston.1
 
 MAN_SUBSTS = \
+	-e 's|__weston_native_backend__|$(WESTON_NATIVE_BACKEND)|g' \
 	-e 's|__weston_modules_dir__|$(pkglibdir)|g' \
 	-e 's|__version__|$(PACKAGE_VERSION)|g'
 

man/weston.man

@@ -98,7 +98,7 @@ Load
 instead of the default backend. The file is searched for in
 .IR "__weston_modules_dir__" ,
 or you can pass an absolute path. The default backend is 
-.I drm-backend.so
+.I __weston_native_backend__
 unless the environment suggests otherwise, see
 .IR DISPLAY " and " WAYLAND_DISPLAY .
 .TP