Add clear GPL license header to missing scripts like autobuild

[ann0see]

Describe the bug

Currently there's no license header in the autobuild scripts. At the beginning when I created the first script, I assumed that it's clear that they are under the same license as the source code files (GPL v2+). After some research and reading Debian discussions, this doesn't seem to be the case. This we should add missing license headers to the autobuild scripts. There are multiple authors, so I think at least the main contributors (@hoffie, @henkdegroot,???) should give an ok.

To Reproduce

See autobuild and tools scripts

Expected behavior

Licensing should be clear in all files

Screenshots

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Operating system

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Version of Jamulus

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Additional context

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[pljones]

At the beginning when I created the first script, I assumed that it's clear that they are under the same license as the source code files (GPL v2+).

Generally, it would be argued I think, that therefore the correct licence is "the same license as the source code files (GPL v2+)". But you do need to state it to ensure this.

[henkdegroot]

Sounds like something that should be done, only contributed a little to these.