diff --git a/CMakeLists.txt b/CMakeLists.txt
index a54e2c6e..6ec8c492 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,7 +22,9 @@ ELSE ()
     MESSAGE (FATAL_ERROR "Invalid PYTHON_DESIRED value: " ${PYTHON_DESIRED})
 ENDIF()
 
-EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c "from sys import stdout; from sysconfig import get_path; stdout.write(get_path('purelib'))" OUTPUT_VARIABLE PYTHON_INSTALL_DIR)
+IF (not PYTHON_INSTALL_DIR)
+    EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c "from sys import stdout; from sysconfig import get_path; stdout.write(get_path('purelib'))" OUTPUT_VARIABLE PYTHON_INSTALL_DIR)
+ENDIF()
 MESSAGE(STATUS "Python install dir is ${PYTHON_INSTALL_DIR}")
 
 SET (SYSCONFDIR /etc)