We really should be documenting these in an easy to find and reference
place.

Because we are racing here. In reality *all* of the java in that target
may rely on the generated file, so we need to block all of them, like we
would for headers in C/C++.

Otherwise, passing result of custom_target() to jar() target is ignored
and won't be compiled resulting in build fail.

To allow the javac -implicit:class behaviour to know where to find
generated .java files then the build directory for the target is also
added to the -sourcefile path.

Although only one file is passed to javac at a time, if your code has
any inter-file dependencies javac still needs to know how to find other
source files for its -implicit:class feature to work whereby it will
automatically also compile files that the given file depends on.

-implicit:class is the default, practical, behaviour of javac since
otherwise it would be necessary to declare the class dependencies
for parallel java builds to be feasible.

Passing "include_directory: include_directory('.')" to jar() causes
-souredir <path/to/top/of/java/src> to be passed to javac which then
enables your source code to have inter-file class dependencies -
assuming none of your source code is generated.

This ensures that '.' is included by default.