patch 8.0.1451: difficult to set the python home directories properly

Problem:    It is difficult to set the python home directory properly for
            Python 2.7 and 3.5 since both use $PYTHONHOME.
Solution:   Add the 'pythonhome' and 'pythonthreehome' options. (Kazuki
            Sakamoto, closes #1266)
diff --git a/src/if_python.c b/src/if_python.c
index f9fc5e1..ab9c854 100644
--- a/src/if_python.c
+++ b/src/if_python.c
@@ -912,6 +912,8 @@
 }
 #endif
 
+static char *py_home_buf = NULL;
+
     static int
 Python_Init(void)
 {
@@ -929,10 +931,15 @@
 	}
 #endif
 
+	if (*p_pyhome != NUL)
+	{
+	    /* The string must not change later, make a copy in static memory. */
+	    py_home_buf = (char *)vim_strsave(p_pyhome);
+	    if (py_home_buf != NULL)
+		Py_SetPythonHome(py_home_buf);
+	}
 #ifdef PYTHON_HOME
-# ifdef DYNAMIC_PYTHON
-	if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
-# endif
+	else if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
 	    Py_SetPythonHome(PYTHON_HOME);
 #endif