Add alt code to 2004/gavin for QEMU

Based on the remarks for QEMU one has to have the macro K defined as 1
so in the alt code it does an #undef K and then a #define K 1. The other
macros are defined if not defined (#ifndef .. #endif).

The instructions could be improved upon. As of now it only tells how to
compile it and that use is similar to the main entry except that one
uses QEMU instead. The file names like kernel, vi etc. are still those
names with the alt version build. I have no good way to test this out
but unless I made a typo in the Makefile everything should be good
assuming that it still works.

This should (if all is good with the Makefile rules) complete
2004/gavin.

2004/gavin/README.md

@@ -5,6 +5,11 @@ make
 ```
 
 
+There is an alternate version for those who wish to use QEMU rather than having
+to reboot and rely on having a floppy drive (if you even remember what those are
+:-) ) etc. See [alternate code](#alternate-code) below.
+
+
 ### Bugs and (Mis)features:
 
 The current status of this entry is:
@@ -73,6 +78,26 @@ vi README.md
 Press and hold any key.
 
 
+## Alternate code:
+
+This version's main difference is that the macro `K` is redefined to `1` so that
+you can use it with `QEMU`.
+
+
+### Alternate build:
+
+```sh
+make alt
+```
+
+### Alternate use:
+
+The use is mostly the same as `gavin` except that one initially executes `gavin.alt`
+(which `make alt` will do) and one will have to use `QEMU` instead. The files
+generated are the same names. See [to use](#to-use) and [try](#try) above as
+well as the judges' remarks below plus [gavin.md](gavin.md).
+
+
 ## Judges' remarks:
 
 Over the years, we've seen a program that immediately dumps core if executed on
@@ -86,12 +111,13 @@ future contests.
 If you do not want to mess with a floppy and you use GRUB, see
 [gavin.md](gavin.md).
 
-You can put additional text files in `fs.tar` for browsing with vi.  If you do
-not want to bother rebooting your computer at all, see <http://bellard.org> for
-QEMU ([Fabrice Bellard](/winners.html#Fabrice_Bellard) is an IOCCC 2001 winner),
-but your experience will be limited; replace `-DK=0` with `-DK=1` in the
-[Makefile](Makefile), and you will have to move the mouse to trigger the initial
-screen update.
+You can put additional text files in `fs.tar` for browsing with vi.
+
+If you do not want to bother rebooting your computer at all, see
+<http://bellard.org> for QEMU ([Fabrice Bellard](/winners.html#Fabrice_Bellard)
+is an IOCCC 2001 winner), but your experience will be limited; use the
+[alternate code](#alternate-code) instead. You'll have to move the mouse to
+trigger the initial screen update.
 
 The judges were able to write a few more programs to run in this OS.
 What are the limitations for such programs?