With 7476614 I've added a boolean so every Compiler implementation can confirm they support incremental compilation.
I now have my doubts if this should be a boolean, maybe an int makes more sense.
With an int a Compiler can define for which arguments they support incremental compilation.
If one sourcefile has changed, the compiler should be able to detect which other sourcefiles need to be compiled.
But also if a classpath/modulepath entry has changed, the compiler should be able to detect which sourcefiles need to be compiled.
I think these are 2 different types and maybe a compiler supports only 1 of these.
By having an integer and refering to javac arguments we can have a more finegrained definition.

e.g.

public static final int SOURCEPATH = 1
public static final int CLASSPATH = 2
public static final int MODULEPATH = 4
public static final int PROCESSORPATH = 8
public static final int PROCESSORMODULEPATH = 16

and incrementalCompilationSupport would return the sum of supported features. That way the maven-compiler-plugin can decide if partial cleaning up is required.

Does this make sense?