Clean up prefer_system and prefer_system_check

[olantwin]

Following on in discussion in #45.

For many packages these are completely outdated.

• slc5 can be dropped
• slc6 maybe as well
• Maybe we can add ubuntu/fedora where appropriate.

For the checks, they are currently very hard to read, so we could make use of YAML multi-line values to make them easier to follow and modify (see e.g. cleanup in #48 )

[olantwin]

Documentation of prefer_system:

prefer_system: a regular expression for architectures which should use the prefer_system_check by default to determine if the system version of the tool can be used. When the rule matches, the result of prefer_system_check determines whether to build the recipe. When the rule does not match, the check is skipped and the recipe is run. Using the switch --always-prefer-system runs the check always (even when the regular expression for the architecture does not match).

[olantwin]

@ThomasRuf : what does the architecture string look like for ubuntu? You can find it in e.g. the subdirectory of sw/ containing the built packages.

[ThomasRuf]

ubuntu2004_x86-64

[olantwin]

Easiest would probably be to have .* for all packages where we want to check the system, and then exclude individual architectures as needed.

Currently playing around in https://regex101.com/ (using the Python regex setting) to test a few options. I think there might actually be a bug in the (?!slc5|slc6) regex, as it seems to match even slc5 and slc6...

Here the architecture formats I am aware of:

fedora31_x86-64
fedora30_x86-64
slc7_x86-64
slc6_x86-64
slc5_x86-64
ubuntu2004_x86-64

[ThomasRuf]

What would be against the opposite approach, exclude only OS which we know don't work?

[olantwin]

Sorry if I was unclear, but that was what I was suggesting (and I think is also what (?!slc5) was supposed to accomplish).