Fix build on os x 10.7

Quassel's cmakelist assumed that all apples had notification center, but it was added in 10.8.  This fixes that assumption.  It's a little strange because cmake just provides the darwin version, and not the os x version, but it works on 10.7 (Should be tested by someone that has 10.8).

Also, some incidental drive-by warning fixing in the second commit, com.trolltech->org.qtproject.