Don't use hard TABs in the makefile snippet in the docs

This doesn't pass the indentation style check and TABs are likely to get
lost when copying/pasting anyhow, so remove them and add an explicit
note about TABs being needed here instead.

docs/gtk/install.md

@@ -264,7 +264,9 @@ which may be combined into a single invocation:
         g++ myfoo.cpp `wx-config --cxxflags --libs` -o myfoo
 
 A simple makefile for a program using wxWidgets could be written
-in the following way:
+in the following way (note that if you are copying and pasting
+this into your makefile, the leading spaces must be replaced by a
+`TAB` character):
 
 ```make
 program: program.o