libpng system package is broken #584
Description
Currently, trying to build the libpng package gives:
$ ./mason build libpng system
The MASON_PKGCONFIG_FILE variable not found in script.sh. Please either provide this variable or override the mason_cflags function hook
This is likely fallout that went unnoticed until now from #356.
We could fix by following the lead of the zlib system package. Or we could remove this package since I don't think libpng system is used.