configure: javac 1.8 does not have --add-exports

... hence it will print a pointless warning about some problem that
doesn't affect JDK 8 anyway.

Change-Id: Ic7669950dbb16815c85e4b8eb9d07d75c68d1b82
Reviewed-on: https://gerrit.libreoffice.org/72172
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

configure.ac

@@ -7516,7 +7516,7 @@ if test -n "$ENABLE_JAVA" -a -z "$JAVAINC"; then
 fi
 SOLARINC="$SOLARINC $JAVAINC"
 
-if test "$ENABLE_JAVA" != "" -a "x" != "x$JAVACOMPILER"; then
+if test "$ENABLE_JAVA" != "" -a "x" != "x$JAVACOMPILER" -a "$_jdk_ver" -ge 10900; then
     url_check_unknown=0
     java_base="testurlcheck"
     java_src="${java_base}.java"