diff3 uses system or nonexistent diff when setting program_prefix

[Jeremy Brinkley]

Using diffutils-2.8.1.

The configure script puts the following in config.h:

#define DEFAULT_DIFF_PROGRAM "diff"

However, if you've specified a --program-prefix or --program-transform-name
(e.g. --program-prefix=g), it won't match (causing gdiff3 to fail because diff
either doesn't exist or doesn't expect the options it passes). The fix is to
induce configure to apply program_transform_name to diff. I modified the
configure script so:

3724,3725c3724,3725
< cat >>confdefs.h <<\_ACEOF
< #define DEFAULT_DIFF_PROGRAM "diff"
---
> cat >>confdefs.h <<_ACEOF
> #define DEFAULT_DIFF_PROGRAM \"`echo diff | sed -e 
> "$program_transform_name"`\"

Note the unquoted here-document delimiter--this is required for command
interpolation so that the sed program can be applied.

I'm not savvy enough with autoconf to make the right change to configure.ac,
unfortunately.

I do understand the --diff-program option allows you to specify a different
diff, but in my opinion the reasonable default is to use GNU diff if you're
installing diffutils.

--
   Jeremy Brinkley