Automatic source filename computation

[Akim Demaille]

The fact that Automake infers some source file names from

    check_PROGRAMS = test-foo

is really nice.  Nevertheless, it is nice to people who program in C,
and less to other languages.  Couldn't we look for test-foo.EXT with
EXT ranging a well defined series instead of the hard coded `c'?

I know it's a bit of magic, and it could be compared to globbing
support.  I find it weaker though, so more admissible.  An alternative
would be a DEFAULT_EXTENSION variable, that's more controlled.