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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