Here's a little patch to GNU diffutils, obtained from the FreeBSD source tree, which makes life easier by honoring the DIFF_OPTIONS environment variable, placing its contents before any options specified on the diff(1) command line. For more information, see revision 650 in my Subversion repository.
Here's a Debian package of diffutils-2.8.1-11.1, which is actually diffutils-2.8.1-11 with the DIFF_OPTIONS patch applied. For more information, see revisions 651 to 657 in my Subversion repository.
Contact: Peter Pentchev <roam@ringlet.net>