build: Use a property to provide an URL for dependency downloading

This patch adds baseUrl property to store a dependency downloading
path. The property may be set for each downloading task separately.

dependencies/build.gradle

@@ -37,6 +37,7 @@ abstract class NativeDep extends DefaultTask {
     }
 
     protected final String host = getCurrentHostTarget();
+    String baseUrl = "https://jetbrains.bintray.com/kotlin-native-dependencies"
 
     @Input
     abstract String getFileName()
@@ -45,10 +46,6 @@ abstract class NativeDep extends DefaultTask {
         return "$baseUrl/$fileName"
     }
 
-    protected String getBaseUrl() {
-        return "https://jetbrains.bintray.com/kotlin-native-dependencies"
-    }
-
     protected File getBaseOutDir() {
         final File res = project.rootProject.file("dist/dependencies")
         res.mkdirs()