Fix install.sh if clause

[sebix]

== with wildcard only works in bash, [[ is also bash-only, not bourne-shell which is used in this script.
To compare with wildcard in bourne-shell we can use the case statement

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[jdelglise]

Same issue encountered here, fixed by replacing #!/bin/sh by #!/bin/bash on my side

[sebix]

Switching to bash would have been the other solution, but that requires bash, probably adding a new dependency (which is often available but not on minimal installations). I therefore opted to stick with the bourne shell. But it's up to the maintainers to decide.