[build] Allow selection of relevant netty-native-transport from CLI

The `forceTranport` property can be used to choose between `nio` and
`native` when running Gradle tasks like tests. This allows to select
the underlying transport to active, eg. when running a test matrix in
CI.

build.gradle

@@ -252,8 +252,32 @@ configure(rootProject) { project ->
 
 	compile "io.netty:netty-handler:${nettyVersion}"
 	compile "io.netty:netty-codec-http:${nettyVersion}"
-	compile "io.netty:netty-transport-native-epoll:${nettyVersion}:linux-x86_64"
-	optional "io.netty:netty-transport-native-kqueue:${nettyVersion}"
+
+	//transport resolution: typical build forces epoll but not kqueue transitively
+	//on the other hand, if we want to make transport-specific tests, we'll make all
+	// native optional at compile time and add correct native/nio to testRuntime
+	if (project.hasProperty("forceTransport")) {
+	  //so that the main code compiles
+	  optional "io.netty:netty-transport-native-epoll:${nettyVersion}"
+	  optional "io.netty:netty-transport-native-kqueue:${nettyVersion}"
+	  //now we explicitly add correctly qualified native, or do nothing if we want to test NIO
+	  if (forceTransport == "native") {
+		if (osdetector.os == "osx") {
+		  testRuntime "io.netty:netty-transport-native-kqueue:${nettyVersion}${os_suffix}"
+		}
+		else if (osdetector.os == "linux") {
+		  testRuntime "io.netty:netty-transport-native-epoll:${nettyVersion}${os_suffix}"
+		}
+	  }
+	  else if (forceTransport != "nio") {
+		throw new InvalidUserDataException("invalid -PforceTranport option " + forceTransport + ", should be native|nio")
+	  }
+	}
+	else {
+	  //classic build to be distributed
+	  compile "io.netty:netty-transport-native-epoll:${nettyVersion}:linux-x86_64"
+	  optional "io.netty:netty-transport-native-kqueue:${nettyVersion}"
+	}
 
 	// Testing
 	testCompile "com.fasterxml.jackson.core:jackson-databind:$jacksonDatabindVersion"