add etag support and if-none-match/if-modified-since to cocoa http handling

common/linux.cpp

@@ -0,0 +1,12 @@
+#include <mbgl/platform/platform.hpp>
+
+namespace mbgl {
+namespace platform {
+
+// Returns the path to the default cache database on this system.
+std::string defaultCacheDatabase() {
+    return "/tmp/mbgl-cache.db";
+}
+
+}
+}