New `JPEGXL_ENABLE_TOOLS` flags (default enabled) lets the user disable
cjxl/djxl tools and the new `JPEGXL_ENABLE_JNI` lets the user disable
building the JNI wrappers.

A few intermediate libraries that should  be only compiled if the tools
that need them are also compiled are now marked as EXCLUDE_FROM_ALL.
This allows user to only compile the jxl libraries.

Fixes #421