Enabling builds with ICU support

[Rapptz]

For a project I am developing, I needed ICU support with the FTS3 module due to the inadequacies in the original tokenizer. However, this requires a separate library (ICU) on top of sqlite3 so it isn't possible to just define it within the amalgamation like the documentation proposes.

I'm unsure what's the proper way to resolve this. One way is to have the ICU builds in the pre-built binaries (Android does this since API 21). The other way is to let the user do it by allowing passing linker flags (for includes and libraries/their paths).

[JoshuaWise]

One way is to compile the extension yourself and load it with db.loadExtension().