libpng.pc, gdlib.pc, zlib.pcetc
are needed in the calls of pkgconfig.parse in src/module_list.py

some perfectly fine implementations of zlib etc. don't install zlib.pc etc. (e.g. MacOS does not).

Since #26286 this problem was sleeping, until we updated pkgconfig on #28883, which made it throw an error on absense of zlib.pc.

This ticket provides generation of zlib.pc and libpng.pc, among with some fly-by refactoring.

CC: @kiwifb @embray @isuruf @mkoeppe

Component: build: configure

Author: Dima Pasechnik

Branch/Commit: u/dimpase/packages/missingpcs @ ba7d963

Issue created by migration from https://trac.sagemath.org/ticket/28906