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-install.sh

@@ -0,0 +1,76 @@
+#!/bin/sh -e
+export LC_ALL=C
+cd "$MESON_INSTALL_DESTDIR_PREFIX"
+msys2_root=$1
+
+# Support running directly from within MSYS2 on Windows.
+if [ -n "$MSYSTEM" ]
+then
+	wine64() { "$@"; }
+	awk() { command awk -v RS='\r?\n' "$@"; }
+fi
+
+# Copy binaries we directly or indirectly depend on.
+cp -p "$msys2_root"/bin/*.dll .
+cp -p "$msys2_root"/bin/wperl.exe .
+cp -p "$msys2_root"/bin/exiftool .
+# The console helper is only useful for debug builds.
+cp -p "$msys2_root"/bin/gspawn-*-helper*.exe .
+cp -pR "$msys2_root"/etc/ .
+
+mkdir -p lib
+cp -pR "$msys2_root"/lib/gdk-pixbuf-2.0/ lib
+cp -pR "$msys2_root"/lib/perl5/ lib
+mkdir -p share/glib-2.0/schemas
+cp -pR "$msys2_root"/share/glib-2.0/schemas/*.Settings.* share/glib-2.0/schemas
+mkdir -p share
+cp -pR "$msys2_root"/share/mime/ share
+mkdir -p share/icons
+cp -pR "$msys2_root"/share/icons/Adwaita/ share/icons
+mkdir -p share/icons/hicolor
+cp -p "$msys2_root"/share/icons/hicolor/index.theme share/icons/hicolor
+
+# Remove unreferenced libraries.
+find lib -name '*.a' -exec rm -- {} +
+awk 'function whitelist(binary) {
+	if (seen[binary]++)
+		return
+
+	delete orphans[binary]
+	while (("strings -a \"" binary "\" 2>/dev/null" | getline string) > 0)
+		if (match(string, /[-.+_a-zA-Z0-9]+[.][Dd][Ll][Ll]$/))
+			whitelist("./" substr(string, RSTART, RLENGTH))
+} BEGIN {
+	while (("find . -type f -path \"./*.[Dd][Ll][Ll]\"" | getline) > 0)
+		orphans[$0]++
+	while (("find . -type f -path \"./*.[Ee][Xx][Ee]\"" | getline) > 0)
+		whitelist($0)
+	while (("find ./lib -type f -path \"./*.[Dd][Ll][Ll]\"" | getline) > 0)
+		whitelist($0)
+	for (library in orphans)
+		print library
+}' | xargs rm --
+
+# Removes unused icons from the Adwaita theme. It could be even more aggressive,
+# since it keeps around lots of sizes and all the GTK+ stock icons.
+find share/icons/Adwaita -type f | awk 'BEGIN {
+	while (("grep -aho \"[a-z][a-z-]*\" *.dll *.exe" | getline) > 0)
+		good[$0] = 1
+} /[.](png|svg|cur|ani)$/ {
+	# Cut out the basename without extensions.
+	match($0, /[^\/]+$/)
+	base = substr($0, RSTART)
+	sub(/[.].+$/, "", base)
+
+	# Try matching while cutting off suffixes.
+	# Disregarding the not-much-used GTK_ICON_LOOKUP_GENERIC_FALLBACK.
+	while (!(keep = good[base]) &&
+		sub(/-(ltr|rtl|symbolic)$/, "", base)) {}
+	if (!keep)
+		print
+}' | xargs rm --
+
+wine64 "$msys2_root"/bin/glib-compile-schemas.exe share/glib-2.0/schemas
+
+# This may speed up program start-up a little bit.
+wine64 "$msys2_root"/bin/gtk-update-icon-cache-3.0.exe share/icons/Adwaita