I had to copy some config_settings out of //bazel/common_config_settings
because these are now treated as separate entities and cannot
see that file.
For libpng, note that we use a genrule to create the
pnglibconf.h instead of pointing to one somewhere else.
This ended up being easier than other things I tried.
Another approach would be to not depend on the version
in third_party/externals, but to clone it via
new_git_repository [1] and apply a patch that creates
the configuration file.
[1] https://bazel.build/rules/lib/repo/git#new_git_repository
Bug: skia:12541
Change-Id: I9a284775dc0f2bdabb145518d5f0803c74fb99fa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/545368
Reviewed-by: Ben Wagner <bungeman@google.com>
