flatbuffers: use script enhancements
flatbuffers: use script enhancements

* rename FLATBUFFERS_FLATC to FLATBUFFERS_FLATC_EXECUTABLE
  * this is the name BuildFlatBuffers.cmake expects
* provide xpFlatBuffersBuild() cmake function
  * wrapper function around build_flatbuffers (from BuildFlatBuffers.cmake)
* issue https://github.com/smanders/externpro/issues/283

smanders/externpro