"make check" still succeeds when there are test failures

[Bruno Haible]

Currently, when DejaGnu is installed, "make check" reports the
number of unexpected failures in its summary, but the exit code
of "make check" is still 0.

It would be more useful if the exit code was set to 1 if there
are unexpected failures, because
  - This is the traditional way a test suite is meant to behave.
  - Then the CI runs would send a failure email, and I could notify you.

I can also make the CI send a failure email without the change,
by grepping the log for the string "unexpected failures". But that
feels like a half-baken approach.

Bruno