Modernizing CMake files for GNURadio 3.7.4

I updated all the CMake-related files based on the GNURadio 3.7.4 module template (`gr_modtool newmod gr-ais`). This allowed me to build gr-ais on OSX using the MacPorts GNURadio install (At least after I got [my environment](https://gist.github.com/johnboiles/c8139c8869306334550b) in order).