GH-138800: fix variable substitution in python3.pc for Android

[thunder-coding]

$(BLDLIBRARY) substitution works in Makefiles where variables are expanded at the time of execution. This expansion does not happen in static files like .pc, so use value of BLDLIBRARY instead of trying to substitute the value.

Patch stolen from downstream distribution of Python for Termux (Android): termux/termux-packages#27739

Preferably needs to be backported to both Python 3.13 as well as Python 3.14, along with the main branch

Fixes #138800

• Issue: .pc for Android contains invalid syntax #138800

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• yakshbari4@gmail.com

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[thunder-coding]

I'd be happy to sign the CLA, but not happy with providing Python's GitHub app with API access to act on my behalf. I'm attaching a PGP signed message, please consider it as my signed version of CLA:

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[emmatyping]

@thunder-coding if you do not want to sign the CLA with CLA bot you can fill out the form here: https://www.python.org/psf/contrib/contrib-form/

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[thunder-coding]

Oops, looks like I did a rebase which has managed to pinged a lot of people. Apologies for the mess I created. I can start up with another PR in order to reduce the spam I unintentionally created.

And I've signed the CLA on https://www.python.org/psf/contrib/contrib-form/, but doesn't seem to have been updated here yet