compile java files with ant for incremental builds

Meson doesn't support incremental builds for java, so delegate the java
compilation to Ant if available.

src/api-impl/build.xml

@@ -0,0 +1,19 @@
+<project name="api-impl" default="dist">
+	<!-- the Ant build is intended to be started from Meson. The properties will be set using commandline parameters -->
+
+	<target name="compile">
+		<mkdir dir="${build}"/>
+		<javac srcdir="${srcdir}" includes="${includes}" destdir="${build}" includeantruntime="false" debug="true">
+			<compilerarg line="${javaargs}"/>
+		</javac>
+	</target>
+
+	<target name="dist" depends="compile">
+		<jar jarfile="${jarfile}" basedir="${build}"/>
+	</target>
+
+	<target name="clean">
+		<delete dir="${build}"/>
+		<delete dir="${dist}"/>
+	</target>
+</project>