Restore --system-site-packages when system virtualenv gets 1.7 upgrade

--- xulrunner-15.0/mozilla/configure.in~	2012-08-29 11:06:34.517985819 +0200
+++ xulrunner-15.0/mozilla/configure.in	2012-08-29 11:23:47.234616054 +0200
@@ -9069,7 +9069,7 @@
 AC_MSG_RESULT([Creating Python virtualenv])
 rm -rf _virtualenv
 mkdir -p _virtualenv
-MACOSX_DEPLOYMENT_TARGET= PYTHONDONTWRITEBYTECODE= $PYTHON $_topsrcdir/python/virtualenv/virtualenv.py --system-site-packages ./_virtualenv
+MACOSX_DEPLOYMENT_TARGET= PYTHONDONTWRITEBYTECODE= /usr/bin/virtualenv ./_virtualenv
 case "$host_os" in
 mingw*)
     PYTHON=`pwd`/_virtualenv/Scripts/python.exe
--- xulrunner-15.0/mozilla/configure~	2012-08-29 11:25:28.121279162 +0200
+++ xulrunner-15.0/mozilla/configure	2012-08-29 11:27:27.981274936 +0200
@@ -25468,7 +25468,7 @@
 echo "$ac_t""Creating Python virtualenv" 1>&6
 rm -rf _virtualenv
 mkdir -p _virtualenv
-MACOSX_DEPLOYMENT_TARGET= PYTHONDONTWRITEBYTECODE= $PYTHON $_topsrcdir/python/virtualenv/virtualenv.py --system-site-packages ./_virtualenv
+MACOSX_DEPLOYMENT_TARGET= PYTHONDONTWRITEBYTECODE= /usr/bin/virtualenv ./_virtualenv
 case "$host_os" in
 mingw*)
     PYTHON=`pwd`/_virtualenv/Scripts/python.exe