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>