libpng and proper pkg-config usage to find where zlib header and libraries are located

Hi,

I have some build scripts in my github repo for building various
dependencies locally for situations where I cannot alter a system's
main configuration to install packages.

One of these build scripts builds libpng from the libpng git repo.
But, to get it to work, I have to man-handle CPPFLAGS and LDFLAGS
variables on the ./configure command line as shown here:

https://github.com/bgoodr/build-locally/blob/955653559800c8bb984ec7a0f75036cb6f324ccf/packages/libpng/linux/build.bash#L113

That is a workaround IMO: Since the libpng build generates its own
libpng*.pc files, shouldn't it also try to use pkg-config to find the
include and library directories produced by zlib which also produces
its own zlib.pc file?

Thanks,
Brent

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