Roll libexpat to 2-4-1

https: //chromium.googlesource.com/external/github.com/libexpat/libexpat.git/+log/e976867fb57a0cd87e3b..a28238bdeebc087
Change-Id: I973d2e342046079e7219110f103021fd06142c2c

Due to "Resolve macro HAVE_EXPAT_CONFIG_H" 8d1bd6ff2c09 it is now
required to provide an expat_config.h. A minimal and maximally portable
one is added.

Change-Id: I973d2e342046079e7219110f103021fd06142c2c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/412776
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
3 files changed