Add missing Makefile.in files from Opus distribution

[softins]

Short description of changes

Two instances of Makefile.in were accidentally omitted from the Opus sources in #3409 due to a rule in .gitignore.
This commit also updates .gitignore not to ignore them.

Makefile.in is not required when building Jamulus from Jamulus.pro, but is needed if experimenting with ./configure in the Opus directory to try out new configuration options.

CHANGELOG: Internal: Add missing Makefile.in files to Opus 1.5.2 distro.

Context: Fixes an issue?

No, but was discovered while reviewing #3487

Does this change need documentation? What needs to be documented and how?

No

Status of this Pull Request

Ready to merge. No effect on current builds.

What is missing until this pull request can be merged?

Only review

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[pljones]

Should we build the OPUS library using its own build system, rather than using Jamulus.pro directly? Is there a platform-independent way to do that? (That's where including it in Jamulus.pro is most likely to be the best way, I guess...)

[softins]

Should we build the OPUS library using its own build system, rather than using Jamulus.pro directly? Is there a platform-independent way to do that? (That's where including it in Jamulus.pro is most likely to be the best way, I guess...)

Certainly worth exploring, although I don't know how platform independent the configure stuff is.

But getting these missing files in, that were overlooked in #3409 is an essential first step.