[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/setup.ant.xml

@@ -24,6 +24,9 @@
 This buildfile is part of projectlombok.org. It sets up the build itself.
 	</description>
 	
+	<!-- increment this number to force a clean of the 'build' dir -->
+	<property name="build.version" value="1" />
+	
 	<property name="pattern.jdk11plus" value="^(\d\d\d+|1[1-9]|[2-9]\d)(\..*)?$" />
 	<property name="ivy.retrieve.pattern" value="lib/[conf]/[organisation]-[artifact].[ext]" />
 	<property environment="env" />
@@ -84,6 +87,37 @@ This buildfile is part of projectlombok.org. It sets up the build itself.
 	
 	<target name="setup.ssh" depends="-setup.ssh.ask, -setup.ssh.save" />
 	
+	<target name="-autoclean.check">
+		<available type="dir" file="build" property="existingbuild.present" />
+		<loadresource property="existingbuild.ver">
+			<first count="1">
+				<resources>
+					<restrict>
+						<fileset dir="." includes="build/build-ver.txt" />
+						<exists />
+					</restrict>
+					<string>0</string>
+				</resources>
+			</first>
+		</loadresource>
+		<condition property="existingbuild.mismatch">
+			<and>
+				<isset property="existingbuild.present" />
+				<not><equals arg1="${existingbuild.ver}" arg2="${build.version}" trim="true" /></not>
+			</and>
+		</condition>
+	</target>
+	
+	<target name="autoclean" depends="-autoclean.check" if="existingbuild.mismatch" description="Checks if a change in the build or deplist neccessitates a clean">
+		<echo>build ver has been incremented, neccessitating a clean...</echo>
+		<delete dir="build" />
+	</target>
+	
+	<target name="-setup.build" depends="autoclean">
+		<mkdir dir="build" />
+		<echo file="build/build-ver.txt">${build.version}</echo>
+	</target>
+	
 	<target name="clean" description="Removes all generated files.">
 		<delete dir="build" quiet="true" />
 	</target>