lib/connections: Add noquic tag

The QUIC package is notorious for being incompatible with either too
old or too new Go releases. Currently it doesn't build with Go 1.15 RC
and I want to test the rest with Go 1.15. With this I can do `go run
build.go --tags noquic` to do that.

lib/connections/quic_dial.go

@@ -4,7 +4,7 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at https://mozilla.org/MPL/2.0/.
 
-// +build go1.12
+// +build go1.12,!noquic
 
 package connections
 

lib/connections/quic_listen.go

@@ -4,7 +4,7 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.12
+// +build go1.12,!noquic
 
 package connections
 

lib/connections/quic_misc.go

@@ -4,7 +4,7 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.12
+// +build go1.12,!noquic
 
 package connections