[master] set DISTCHECK_GTEST_CONFIGURE_FLAG for --with-gtest too

This is for trac #2481. Review via jabber.

configure.ac

@@ -903,11 +903,12 @@ AC_SUBST(MULTITHREADING_FLAG)
 #
 GTEST_LDFLAGS=
 GTEST_LDADD=
-# TODO: set DISTCHECK_GTEST_CONFIGURE_FLAG for --with-gtest too
 DISTCHECK_GTEST_CONFIGURE_FLAG=
 
 if test "x$enable_gtest" = "xyes" ; then
 
+    DISTCHECK_GTEST_CONFIGURE_FLAG="--with-gtest=$gtest_path"
+
     if test -n "$with_gtest_source" ; then
 
           if test "x$GTEST_SOURCE" = "xyes" ; then