acme/autocert: fix build in Go 1.9

Updates golang/go#22066

Change-Id: I7eb6a60deb6680003245815760e2ce6a8f7d8b15
Reviewed-on: https://go-review.googlesource.com/116496
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

acme/autocert/autocert.go

@@ -276,8 +276,9 @@ func supportsECDSA(hello *tls.ClientHelloInfo) bool {
 		ecdsaOK := false
 	schemeLoop:
 		for _, scheme := range hello.SignatureSchemes {
+			var tlsECDSAWithSHA1 tls.SignatureScheme = 0x0203 // constant added in Go 1.10
 			switch scheme {
-			case tls.ECDSAWithSHA1, tls.ECDSAWithP256AndSHA256,
+			case tlsECDSAWithSHA1, tls.ECDSAWithP256AndSHA256,
 				tls.ECDSAWithP384AndSHA384, tls.ECDSAWithP521AndSHA512:
 				ecdsaOK = true
 				break schemeLoop