[build] the build can now autoclean

autocleaning means that the build system detects that the usual incremental mode,
where the build artefacts are not deleted and instead used to skip steps already performed,
is not an option due to changes in the build script itself or a change in deps not detectable.

It works by having a version number which can be incremented, and a system that checks
for mismatches and forces a clean.

buildScripts/build-support.ant.xml

@@ -50,7 +50,7 @@ None of these tasks are normally needed, unless modifying how the build works, s
 		<input addproperty="target.jdk">You need to specify the JDK9+ jdk whose jdk.compiler and java.compiler modules are to be converted. Run ant with -Dtarget.jdk=/full/path/here to automate this, or type the path in now (for example: /Library/JavaVirtualMachines/jdk-14.jdk/Contents/Home):</input>
 	</target>
 	
-	<target name="make.javac-patch-jar" depends="-ask.target-jdk" description="to test javac13 on JDK14, for example, you need a jar (not a jmod), to use with --patch-modules. This task makes those.">
+	<target name="make.javac-patch-jar" depends="-ask.target-jdk, -setup.build" description="to test javac13 on JDK14, for example, you need a jar (not a jmod), to use with --patch-modules. This task makes those.">
 		<exec executable="${target.jdk}/bin/java${executable.suffix}" outputproperty="target.javac.version.full" errorproperty="target.javac.version.err">
 			<arg value="--version" />
 		</exec>