Adds support for the LITE_RUNTIME optimization option to the protobuf-c
compiler. Enabling this option would generate lighter weight message,
enum, and service descriptors that contain NO strings. As a result,
calls to lookup descriptors via the *_get_{field,value,method}_by_name
API will return NULL.

Default compiler behavior (when optimize_for is not specified or is not
set to LITE_RUNTIME) is unchanged.