--- a/swig/python/CMakeLists.txt
+++ b/swig/python/CMakeLists.txt
@@ -40,11 +40,13 @@
   OUTPUT_VARIABLE PYTHON_PREFIX
 )
 file ( TO_CMAKE_PATH "${PYTHON_PREFIX}" PYTHON_PREFIX )
+if ( NOT PYTHON_SITE_DIR )
 execute_process (
   COMMAND ${PYTHON_EXECUTABLE} -c
   	"import site, sys; sys.stdout.write(site.getsitepackages()[-1])"
   OUTPUT_VARIABLE PYTHON_SITE_DIR
 )
+endif ( PYTHON_SITE_DIR )
 file ( TO_CMAKE_PATH "${PYTHON_SITE_DIR}" PYTHON_SITE_DIR )
 string ( REGEX REPLACE "^${PYTHON_PREFIX}/" ""
   PYTHON_SITE_DIR "${PYTHON_SITE_DIR}"