Allow to pass JAVA_SOURCE/TARGET_VER into configure
...for those who pretend they know what they are doing (e.g., to experiment with
building against Java 12 which no longer supports Java 1.6 source/target, even
though that results in a build that is not baseline compatible).
Change-Id: I6e8617bc5b90212473652bdfac40bb48e9623a08
Reviewed-on: https://gerrit.libreoffice.org/71218
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
(cherry picked from commit
d365f36331874f94bbd9832fbd19ace90fafefec)