workdir/UnpackedTarball/languagetool/build.xml uses ${source}, ${target}

in its javac invocations, so we apparently need to set those variables instead
of ant.build.javac.source/target (which are documented at
<https://ant.apache.org/manual/javacprops.html> to be supported since Ant 1.7,
but are apparently not used here).  (Found when doing a test build with Java 12
and JAVA_SOURCE/TARGET_VER explicitly configured as 7 with
<https://gerrit.libreoffice.org/#/c/71218/> "Allow to pass
JAVA_SOURCE/TARGET_VER into configure", which caused building
external/languagetool to nevertheless complain about unsupported 1.6
source/target switches.)

Change-Id: I624f77b65ef81bc592a78559e67062a42cfff05a
Reviewed-on: https://gerrit.libreoffice.org/71219
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/languagetool/ExternalProject_languagetool.mk

@@ -26,8 +26,8 @@ $(call gb_ExternalProject_get_state_target,languagetool,build) :
 		$(if $(verbose),-v,-q) \
 		-f build.xml \
 		-Dbuild.label="build-$(LIBO_VERSION_MAJOR).$(LIBO_VERSION_MINOR).$(LIBO_VERSION_MICRO).$(LIBO_VERSION_PATCH)" \
-		-Dant.build.javac.source=$(JAVA_SOURCE_VER) \
-		-Dant.build.javac.target=$(JAVA_TARGET_VER) \
+		-Dsource=$(JAVA_SOURCE_VER) \
+		-Dtarget=$(JAVA_TARGET_VER) \
 		$(if $(debug),-Dbuild.debug="on") \
 		-Dext.ooo.juh.lib="$(call gb_Jar_get_target,juh)" \
 		-Dext.ooo.jurt.lib="$(call gb_Jar_get_target,jurt)" \