Comment 5 for bug 1166506

How about:

for pkg in supp_png_pkgs:
        try:
            ctx.check_cfg(package=pkg, uselib_store='libpng', args = ['--cflags', '--libs'])
        except:
            pass
        else:
            have_png = True
            break

This way we only check for the package once (and get saner config output).