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/configure.ac b/configure.ac
index 7d403cf..ecaea63 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,6 +20,9 @@
 LT_PREREQ([2.2])
 LT_INIT([disable-static])
 
+AC_ARG_VAR([WESTON_NATIVE_BACKEND],
+           [Set the native backend to use, if Weston is not running under Wayland nor X11. @<:@default=drm-backend.so@:>@])
+
 PKG_PROG_PKG_CONFIG()
 
 AC_CHECK_FUNC([dlopen], [],
@@ -255,6 +258,13 @@
 AC_SUBST(GCC_CFLAGS)
 AC_SUBST(GCC_CXXFLAGS)
 
+if test "x$WESTON_NATIVE_BACKEND" = "x"; then
+	WESTON_NATIVE_BACKEND="drm-backend.so"
+fi
+AC_MSG_NOTICE([Weston's native backend: $WESTON_NATIVE_BACKEND])
+AC_DEFINE_UNQUOTED([WESTON_NATIVE_BACKEND], ["$WESTON_NATIVE_BACKEND"],
+		   [The default backend to load, if not wayland nor x11.])
+
 WAYLAND_SCANNER_RULES(['$(top_srcdir)/protocol'])
 
 AC_CONFIG_FILES([Makefile