message: mavlink: install pymavlink dependencies
  This automatically installs the pip packages lxml and future as
  required.
diff: @@ -17,6 +17,8 @@ ExternalProject_add(
  GIT_TAG 0b5b5dabf9c6ae22f2f93f0db9a4e19dcf6f19ac
  PREFIX mavlink
  CONFIGURE_COMMAND Python3::Interpreter
  + -m pip install -r pymavlink/requirements.txt
  + COMMAND Python3::Interpreter
  -m pymavlink.tools.mavgen
  --lang=C
  --wire-protocol=2.0
  