build: repair the build on Windows (#20)

Replicate the behaviour from upstream ICU:
  ```
  #if defined(U_TIMEZONE) || defined(U_HAVE_TIMEZONE)
      /* Use the predefined value. */
  #elif U_PLATFORM == U_PF_ANDROID
  #   define U_TIMEZONE timezone
  #elif defined(__UCLIBC__)
      // uClibc does not have __timezone or _timezone.
  #elif defined(_NEWLIB_VERSION)
  #   define U_TIMEZONE _timezone
  #elif defined(__GLIBC__)
      // glibc
  #   define U_TIMEZONE __timezone
  #elif U_PLATFORM_IS_LINUX_BASED
      // not defined
  #elif U_PLATFORM_USES_ONLY_WIN32_API
  #   define U_TIMEZONE _timezone
  #elif U_PLATFORM == U_PF_BSD && !defined(__NetBSD__)
     /* not defined */
  #elif U_PLATFORM == U_PF_OS400
     /* not defined */
  #elif U_PLATFORM == U_PF_IPHONE
     /* not defined */
  #else
  #   define U_TIMEZONE timezone
  #endif
  ```

With this change, ICU at least builds, but it is unclear if it functions
properly on Windows.

Package.swift

@@ -9,7 +9,19 @@ var buildSettings: [CXXSetting] = [
     .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", to: "_timezone", .when(platforms: [.windows])),
+    .define("U_TIMEZONE", to: "timezone",
+            .when(platforms: [
+                .iOS,
+                .macOS,
+                .tvOS,
+                .watchOS,
+                .macCatalyst,
+                .driverKit,
+                .android,
+                .linux,
+                .wasi
+            ])),
     .define("U_TIMEZONE_PACKAGE", to: "\"icutz44l\""),
     .define("FORTIFY_SOURCE", to: "2"),
     .define("STD_INSPIRED"),