configure: limit JunitTest VM usage

By default JVM will allocate a significant chunk of the available system
VM, but our JunitTest run well for me with just 64 MB of max heap,
so set this as a limit at least for Sun JVMs, to have tests running more
smoothly on OS that lack VM over-commit.

Change-Id: I5e4b5595decc3a133e7e285dfd7463d436fa458d
Reviewed-on: https://gerrit.libreoffice.org/11698Reviewed-by: David Ostrovsky <david@ostrovsky.org>
Tested-by: David Ostrovsky <david@ostrovsky.org>

configure.ac

@@ -6712,6 +6712,9 @@ you must use the "--with-jdk-home" configure option explicitly])
             if test "$_os" = "WINNT"; then
                 JAVA_HOME=`echo $JAVA_HOME | $SED "s,\.[[eE]][[xX]][[eE]]$,,"`
             fi
+
+            # set to limit VM usage for JunitTests
+            JAVAIFLAGS=-Xmx64M
         fi
     else
         AC_MSG_ERROR([Java not found. You need at least jdk-1.5, or gcj-4])