Added a "make verify" target.

This is more than for cosmetic purposes to match how it's done with autoconf.

Due to the fact that we use environment variables to turn off certain backends during the tests, simply running "ctest" or "make test" can result in failed tests.

This is because if you do "EVENT_NOEPOLL=yes && export EVENT_NOEPOLL" and then run the tests, when running the epoll tests, the EPOLL backend will be turned off. There is no way of unsetting an environment variable for a test in CMake, you can only set them. And since libevent simply checks if the environment variable is set (it doesn't check the actual value of it), this won't work.

So to remedy this, we create the "make verify" target that first unsets all the EVENT_NO* environment variables, and then runs ctest.

Also bumped the required CMake version from 2.6 to 2.8, since the set_test_properties(bla PROPERTIES ENVIRONMENT "SOME_VAR") requires 2.8

Added some explicit dependencies for the test programs to libevent, so they don't just fail if you try to run the tests without first doing "make"