Versioning adjustments

Three changes in this pull request:
* During the Debian build process, figure out the version number of the source package we're building, then inject it into the VERSION_INFO within CMake.
 * This adds a new CMake variable, VERSION_INFO_OVERRIDE, which, if set and non-null, bypasses the VERSION_INFO_EXTRA logic in host/cmake/modules/Version.cmake.
* Adds top-level options for toggling host and fx3 builds
 * ENABLE_HOST_BUILD and ENABLE_FX3_BUILD can be set from the cmake command line to avoid building something we don't want or can't build.
* VERSION_INFO_EXTRA is defined in the top-level CMakeLists.txt
 * This adds a new CMake variable, TAGGED_RELEASE, which can be set to 'off' to remove the -git suffix from the component versions.

Fixes: #154