Build “fat binaries” for macOS (#1461)

The same binary, both for Intel and ARM Macs!

Builds libpng manually since the lib Brew distributes only contains code for the host arch.

.github/scripts/build_libpng.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+set -euo pipefail
+
+pngver=1.6.43
+
+## Grab sources and check them
+
+curl -LOJ "http://prdownloads.sourceforge.net/libpng/libpng-$pngver.tar.xz?download"
+# Brew doesn't provide any sha256sum, so we're making do with `sha2` instead.
+if [ "$(sha2 -q -256 libpng-$pngver.tar.xz)" != 6a5ca0652392a2d7c9db2ae5b40210843c0bbc081cbd410825ab00cc59f14a6c ]; then
+	sha2 -256 libpng-$pngver.tar.xz
+	echo Checksum mismatch! Aborting. >&2
+	exit 1
+fi
+
+## Extract sources and patch them
+
+tar -xvf libpng-$pngver.tar.xz
+
+## Start building!
+
+mkdir -p build
+cd build
+../libpng-$pngver/configure --disable-shared --enable-static \
+	CFLAGS="-O3 -flto -DNDEBUG -mmacosx-version-min=10.9 -arch x86_64 -arch arm64 -fno-exceptions"
+make -kj
+make install prefix="$PWD/../libpng-staging"