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

--- a/mozilla/configure.in~	2013-02-15 22:14:45.000000000 +0100
+++ a/mozilla/configure.in	2013-02-20 17:05:55.258742791 +0100
@@ -141,7 +141,7 @@
     PYTHON=`cd $MOZ_BUILD_ROOT && pwd -W`/_virtualenv/Scripts/python.exe
     ;;
 *)
-    PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
+    PYTHON=/usr/bin/python
     ;;
 esac
 
--- a/mozilla/configure~	2013-02-15 22:15:48.000000000 +0100
+++ a/mozilla/configure	2013-02-20 17:06:11.479134735 +0100
@@ -1344,7 +1344,7 @@
     PYTHON=`cd $MOZ_BUILD_ROOT && pwd -W`/_virtualenv/Scripts/python.exe
     ;;
 *)
-    PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
+    PYTHON=/usr/bin/python
     ;;
 esac
 
--- iceweasel-19.0/mozilla/configure.in~	2013-02-20 17:11:35.849980531 +0100
+++ iceweasel-19.0/mozilla/configure.in	2013-02-20 17:18:24.806617099 +0100
@@ -128,13 +128,6 @@
     AC_MSG_ERROR([python was not found in \$PATH])
 fi
 
-AC_MSG_RESULT([Creating Python environment])
-dnl This verifies our Python version is sane and ensures the Python
-dnl virtualenv is present and up to date. It sanitizes the environment
-dnl for us, so we don't need to clean anything out.
-$PYTHON $_topsrcdir/build/virtualenv/populate_virtualenv.py \
-    $_topsrcdir $MOZ_BUILD_ROOT/_virtualenv || exit 1
-
 dnl Create a virtualenv where we can install local Python packages
 case "$host_os" in
 mingw*)
--- iceweasel-19.0/mozilla/configure~	2013-02-20 17:11:35.853313949 +0100
+++ iceweasel-19.0/mozilla/configure	2013-02-20 17:17:59.639339399 +0100
@@ -1335,10 +1335,6 @@
     { echo "configure: error: python was not found in \$PATH" 1>&2; exit 1; }
 fi
 
-echo "$ac_t""Creating Python environment" 1>&6
-$PYTHON $_topsrcdir/build/virtualenv/populate_virtualenv.py \
-    $_topsrcdir $MOZ_BUILD_ROOT/_virtualenv || exit 1
-
 case "$host_os" in
 mingw*)
     PYTHON=`cd $MOZ_BUILD_ROOT && pwd -W`/_virtualenv/Scripts/python.exe