Fix make check warning

Making check in pdns
Makefile:2163: warning: overriding commands for target `check'
Makefile:2023: warning: ignoring old commands for target `check'
Unit tests are not enabled

Automake doesn't like the check target to
be overridden:
http://lists.gnu.org/archive/html/automake/2011-09/msg00028.html
Merge pull request #1593 from rubenk/fix-make-check-warning

Fix make check warning