Allow us to pass in PYTHON_SITE_PACKAGES

The python binary used when building for nativesdk doesn't give us the
correct path here so we need to be able to specify it ourselves.

Upstream-Status: Inappropriate [config]

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

--- a/configure.in
+++ b/configure.in
@@ -743,7 +743,8 @@ dnl
 
 PYTHON_VERSION=
 PYTHON_INCLUDES=
-PYTHON_SITE_PACKAGES=
+# Allow this to be set externally
+#PYTHON_SITE_PACKAGES=
 PYTHON_TESTS=
 pythondir=
 if test "$with_python" != "no" ; then