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>
diff --git a/man/weston.man b/man/weston.man
index 7ccfac8..efb6d2b 100644
--- a/man/weston.man
+++ b/man/weston.man
@@ -98,7 +98,7 @@
 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