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>

configure.ac

@@ -7203,8 +7203,8 @@ dnl Checks for javac
 dnl ===================================================================
 if test "$ENABLE_JAVA" != ""; then
     javacompiler="javac"
-    JAVA_SOURCE_VER="1.6"
-    JAVA_TARGET_VER="1.6"
+    : ${JAVA_SOURCE_VER=1.6}
+    : ${JAVA_TARGET_VER=1.6}
     if test -z "$with_jdk_home"; then
         AC_PATH_PROG(JAVACOMPILER, $javacompiler)
     else