gmic: fix license, clean do_build

[sgn]

Testing the changes

• I tested the changes in this PR: NO

@newbluemoon

[newbluemoon]

I tried not to change too much in the existing template and avoid patching the Makefile and .pro file, but seeing it now it’s a much much cleaner approach! :)
However, as I just tried to fix the build wrt the opencv update and don’t use it, I’m not sure how to test it properly.

[github-actions]

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