Make cross-compilation scripts work from MSYS2

This is weird and runs very slowly.

Meson can also find libraries outside the subroot,
in particular the fast float plugin.

msys2-configure.sh

@@ -0,0 +1,132 @@
+#!/bin/sh -e
+# msys2-configure.sh: set up an MSYS2-based Meson build targeting x86-64.
+# Dependencies: AWK, sed, sha256sum, cURL, bsdtar,
+# wine64, Meson, mingw-w64-binutils, mingw-w64-gcc, pkg-config
+repository=https://repo.msys2.org/mingw/mingw64/
+
+# Support running directly from within MSYS2 on Windows.
+if [ -n "$MSYSTEM" ]
+then
+	wine64() { "$@"; }
+	awk() { command awk -v RS='\r?\n' "$@"; }
+fi
+
+status() {
+	echo "$(tput bold)-- $*$(tput sgr0)"
+}
+
+dbsync() {
+	status Fetching repository DB
+	[ -f db.tsv ] || curl -# "$repository/mingw64.db" | bsdtar -xOf- | awk '
+		function flush() { print f["%NAME%"] f["%FILENAME%"] f["%DEPENDS%"] }
+		NR > 1 && $0 == "%FILENAME%" { flush(); for (i in f) delete f[i] }
+		!/^[^%]/ { field = $0; next } { f[field] = f[field] $0 "\t" }
+		field == "%SHA256SUM%" { path = "*packages/" f["%FILENAME%"]
+			sub(/\t$/, "", path); print $0, path > "db.sums" } END { flush() }
+	' > db.tsv
+}
+
+fetch() {
+	status Resolving "$@"
+	mkdir -p packages
+	awk -F'\t' 'function get(name,    i, a) {
+		if (visited[name]++ || !(name in filenames)) return
+		print filenames[name]; split(deps[name], a); for (i in a) get(a[i])
+	} BEGIN { while ((getline < "db.tsv") > 0) {
+		filenames[$1] = $2; deps[$1] = ""; for (i = 3; i <= NF; i++) {
+			gsub(/[<=>].*/, "", $i); deps[$1] = deps[$1] $i FS }
+	} for (i = 0; i < ARGC; i++) get(ARGV[i]) }' "$@" | while IFS= read -r name
+	do
+		status Fetching "$name"
+		[ -f "packages/$name" ] || curl -#o "packages/$name" "$repository/$name"
+	done
+
+	version=$(curl -# https://exiftool.org/ver.txt)
+	name=exiftool-$version.tar.gz remotename=Image-ExifTool-$version.tar.gz
+	status Fetching "$remotename"
+	[ -f "$name" ] || curl -#o "$name" "https://exiftool.org/$remotename"
+	ln -sf "$name" exiftool.tar.gz
+}
+
+verify() {
+	status Verifying checksums
+	sha256sum --ignore-missing --quiet -c db.sums
+}
+
+extract() {
+	status Extracting packages
+	for subdir in *
+	do [ -d "$subdir" -a "$subdir" != packages ] && rm -rf -- "$subdir"
+	done
+	for i in packages/*
+	do bsdtar -xf "$i" --strip-components 1 mingw64
+	done
+
+	bsdtar -xf exiftool.tar.gz
+	mv Image-ExifTool-*/exiftool bin
+	mv Image-ExifTool-*/lib/* lib/perl5/site_perl
+	rm -rf Image-ExifTool-*
+}
+
+configure() {
+	# Don't let GLib programs inherit wrong paths from the environment.
+	export XDG_DATA_DIRS=$msys2_root/share
+
+	status Configuring packages
+	wine64 bin/update-mime-database.exe share/mime
+	wine64 bin/glib-compile-schemas.exe share/glib-2.0/schemas
+	wine64 bin/gdk-pixbuf-query-loaders.exe \
+		> lib/gdk-pixbuf-2.0/2.10.0/loaders.cache
+}
+
+setup() {
+	status Setting up Meson
+	[ -n "$MSYSTEM" ] && wrap=false || wrap=true
+
+	cat >"$toolchain" <<-EOF
+	[binaries]
+	c = 'x86_64-w64-mingw32-gcc'
+	cpp = 'x86_64-w64-mingw32-g++'
+	ar = 'x86_64-w64-mingw32-gcc-ar'
+	ranlib = 'x86_64-w64-mingw32-gcc-ranlib'
+	strip = 'x86_64-w64-mingw32-strip'
+	windres = 'x86_64-w64-mingw32-windres'
+	pkgconfig = 'pkg-config'
+
+	[properties]
+	sys_root = '$builddir'
+	msys2_root = '$msys2_root'
+	pkg_config_libdir = '$msys2_root/share/pkgconfig:$msys2_root/lib/pkgconfig'
+	needs_exe_wrapper = $wrap
+
+	[host_machine]
+	system = 'windows'
+	cpu_family = 'x86_64'
+	cpu = 'x86_64'
+	endian = 'little'
+	EOF
+
+	meson setup --buildtype=debugoptimized --prefix="$packagedir" \
+		--bindir . --libdir . --cross-file="$toolchain" "$builddir" "$sourcedir"
+}
+
+sourcedir=$(realpath "${2:-$(dirname "$0")}")
+builddir=$(realpath "${1:-builddir}")
+packagedir=$builddir/package
+toolchain=$builddir/msys2-cross-toolchain.meson
+
+# This directory name matches the prefix in .pc files, so we don't need to
+# modify them (pkgconf has --prefix-variable, but Meson can't pass that option).
+msys2_root=$builddir/mingw64
+
+mkdir -p "$msys2_root"
+cd "$msys2_root"
+dbsync
+fetch mingw-w64-x86_64-gtk3 mingw-w64-x86_64-lcms2 \
+	mingw-w64-x86_64-libraw mingw-w64-x86_64-libheif \
+	mingw-w64-x86_64-perl mingw-w64-x86_64-perl-win32-api \
+	mingw-w64-x86_64-libwinpthread-git # Because we don't do "provides"?
+verify
+extract
+configure
+setup