qwt5 and pyqwt5 fail to build on Ubuntu 20.04

[jkbecker]

To reproduce:

• run a clean Ubuntu 20.04 system with current PyBOMBS (such as the ubuntu_2004 test container in gnuradio/pybombs)
• run pybombs install qwt5 pyqwt5 (or just either of the two)

Result

The installer will build an install tree that includes qt4, and fail during the build of qt4.

[argilo]

Is it important for qwt5 and qt4 to be usable on Ubuntu 20.04? Both have been unsupported for at least five years now.

[jkbecker]

Maybe it can fail early with an "unsupported" message on 20.04, rather than attempting a lengthy unsupported build and failing with a bunch of weird compile errors? Not sure if such deliberate "install fails" are a thing PyBOMBS can do / if this makes sense?

[jkbecker]

@argilo I replaced qwt5 with qwt6 in gnuradio/pybombs-docker#2, so as far as I'm concerned this is not a road blocker for me right now. Should we just add a compatibility note to the recipe (as a comment) to preempt confusions as mine above?

[argilo]

Perhaps we could just remove the source: line from qwt5.lwr?