WORKAROUND: Allow to honor PYTHONHOME environment variable for +python/dyn and +python3/dyn

It is temporary fix and it may be changed in the future
vim/vim#500

Example:

	let $PYTHONHOME="/usr/local/Frameworks/Python.framework/Versions/2.7"
	set pythondll=/usr/local/Frameworks/Python.framework/Versions/2.7/Python

Author: splhack

src/if_python.c

@@ -932,7 +932,10 @@ Python_Init(void)
 #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();

src/if_python3.c

@@ -858,7 +858,10 @@ Python3_Init(void)
 
 
 #ifdef PYTHON3_HOME
-	Py_SetPythonHome(PYTHON3_HOME);
+# ifdef DYNAMIC_PYTHON3
+	if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
+# endif
+	    Py_SetPythonHome(PYTHON3_HOME);
 #endif
 
 	PyImport_AppendInittab("vim", Py3Init_vim);