Inject the current Debian snapshot version

This figures out the version number of the source package we're
building, then injects it into the VERSION_INFO within CMake.
Add top-level options for toggling host/fx3 builds

Adds ENABLE_HOST_BUILD and ENABLE_FX3_BUILD as CMake options.
Define VERSION_INFO_EXTRA in top-level CMakeLists

Allows the -git tag to be dropped from all components easily,
without having to edit any files.