Java 1.7 "support" was added on August the 2nd: https://github.com/droolsjbpm/drools/commit/bc259b264a5a088d7704ba319014ce4838cb382e
The only modification was to add 1.7 to the list of allowed Java versions :)
If you want to use java 1.7, then you will need to use the latest snapshot of drools. You can also modify JavaDialectConfiguration.java and EclipseJavaCompilerSettings.java by hand.