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/option.h b/src/option.h
index c9fa4d3..21be3a3 100644
--- a/src/option.h
+++ b/src/option.h
@@ -696,9 +696,15 @@
 #if defined(DYNAMIC_PYTHON3)
 EXTERN char_u	*p_py3dll;	/* 'pythonthreedll' */
 #endif
+#ifdef FEAT_PYTHON3
+EXTERN char_u	*p_py3home;	/* 'pythonthreehome' */
+#endif
 #if defined(DYNAMIC_PYTHON)
 EXTERN char_u	*p_pydll;	/* 'pythondll' */
 #endif
+#ifdef FEAT_PYTHON
+EXTERN char_u	*p_pyhome;	/* 'pythonhome' */
+#endif
 #if defined(FEAT_PYTHON) || defined(FEAT_PYTHON3)
 EXTERN long	p_pyx;		/* 'pyxversion' */
 #endif