hack/libdm_installed: Add a test for libdevmapper.h

Avoid crashing 'make' with:

  No package 'devmapper' found

by disabling the devmapper driver when the library it requires is not
installed.  Also give the libdm_no_deferred_remove script a more
specific name to avoid confusion.

Signed-off-by: W. Trevor King <wking@tremily.us>

hack/libdm_installed.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+cc -E - > /dev/null 2> /dev/null << EOF
+#include <libdevmapper.h>
+EOF
+if test $? -ne 0 ; then
+	echo exclude_graphdriver_devicemapper
+fi