diff --git a/configure.ac b/configure.ac
index 9a4aeb5..2605478 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6,6 +6,12 @@
 
 AM_INIT_AUTOMAKE
 
+AC_ARG_VAR([PY_PREFIX], [Prefix for installing Python modules.
+                         (Normal --prefix is ignored for Python because
+                         Python has different conventions.)
+                         Default = "/usr"])
+AS_IF([test "x$PY_PREFIX" == x], [PY_PREFIX="/usr"])
+
 AC_CONFIG_FILES([Makefile])
 
 AC_CONFIG_SUBDIRS([compiler/cpp lib/cpp lib/py if])