build: indicate that we are building ICU statically

We need to explicitly specify that we are building ICU statically as
this changes the build model for Windows.  Since the products are static
and the targets are built statically, simply assume static builds rather
than dynamic builds.  This allows us to partially build this on Windows.

Author: compnerd

Package.swift

@@ -7,6 +7,7 @@ let buildSettings: [CXXSetting] = [
     .define("DEBUG", to: "1", .when(configuration: .debug)),
     .define("U_SHOW_CPLUSPLUS_API", to: "1"),
     .define("U_SHOW_INTERNAL_API", to: "1"),
+    .define("U_STATIC_IMPLEMENTATION"),
     .define("U_TIMEZONE", to: "timezone"),
     .define("U_TIMEZONE_PACKAGE", to: "\"icutz44l\""),
     .define("FORTIFY_SOURCE", to: "2"),
@@ -101,4 +102,4 @@ fileprivate extension Array {
         me.append(contentsOf: other)
         return me
     }
-}
+}