fix internal links in entry index.html from 1984 thru 2012

2004/gavin/index.html

@@ -228,7 +228,7 @@ necessary work. Applications are passed a pointer to their task structure,
 which is partially defined by the OS, and partially for the application’s own
 use.</p>
 <p>The program compiles into a tool to build a kernel image,
-so having built the program, the <a href="Makefile">Makefile</a> will run it,
+so having built the program, the <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2004/gavin/Makefile">Makefile</a> will run it,
 redirecting the output into a file called <code>kernel</code>.
 The Makefile will then proceed to build a root filesystem image.
 This involves rebuilding the program with different compiler flags,