patch 7.4.1012
Problem:    Vim overwrites the value of $PYTHONHOME.
Solution:   Do not set $PYTHONHOME if it is already set. (Kazuki Sakamoto,
            closes #500)
diff --git a/src/if_python.c b/src/if_python.c
index 9d28df0..94b299e 100644
--- a/src/if_python.c
+++ b/src/if_python.c
@@ -928,7 +928,10 @@
 #endif
 
 #ifdef PYTHON_HOME
-	Py_SetPythonHome(PYTHON_HOME);
+# ifdef DYNAMIC_PYTHON
+	if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
+# endif
+	    Py_SetPythonHome(PYTHON_HOME);
 #endif
 
 	init_structs();