Cache library deps on Windows

Besides the immediate performance improvement, reducing
the frequency of downloads should make spurious
failures (network, rate-limiting...) less bothersome.

.github/scripts/get_win_deps.ps1

@@ -1,18 +0,0 @@
-function getlibrary ([string] $URI, [string] $filename, [string] $hash, [string] $destdir) {
-	$wc = New-Object Net.WebClient
-	[string] $downloadhash = $null
-	try {
-		$wc.DownloadFile($URI, $filename)
-		$downloadhash = $(Get-FileHash $filename -Algorithm SHA256).Hash
-	} catch {
-		Write-Host "${filename}: failed to download"
-		exit 1
-	}
-	if ($hash -ne $downloadhash) {
-		Write-Host "${filename}: SHA256 mismatch ($downloadhash)"
-		exit 1
-	}
-	Expand-Archive -DestinationPath $destdir $filename
-}
-
-getlibrary 'https://github.com/lexxmark/winflexbison/releases/download/v2.5.25/win_flex_bison-2.5.25.zip' 'winflexbison.zip' '8d324b62be33604b2c45ad1dd34ab93d722534448f55a16ca7292de32b6ac135' bison

.github/scripts/install_deps.sh

@@ -7,7 +7,7 @@ case "${1%-*}" in
 		sudo apt-get -qq update
 		sudo apt-get install -yq bison libpng-dev pkg-config
 		;;
-		macos)
+	macos)
 		# macOS bundles GNU Make 3.81, which doesn't support synced output.
 		# We leave it as the default in `PATH`, to test that our Makefile works with it.
 		# However, CMake automatically uses Homebrew's `gmake`, so our CI has synced output.
@@ -20,11 +20,17 @@ case "${1%-*}" in
 	freebsd)
 		pkg install -y bash bison cmake git png
 		;;
+	windows)
+		choco install -y winflexbison3
+		# The below expects the base name, not the Windows-specific name.
+		bison() { win_bison "$@"; } # An alias doesn't work, so we use a function instead.
+		;;
 	*)
 		echo "WARNING: Cannot install deps for OS '$1'"
 		;;
 esac
 
+echo "PATH=($PATH)" | sed 's/:/\n      /g'
 bison --version
 make --version
 cmake --version