undo intermediate Makefile changes of 1996 to 2020

The commit 00ae59cd47 had a
mistake that introduces changes to Makefiles for 1996 thru 2020
that should not have been included.

This change reverses these intermediate changes until
the given year's Makefiles are ready.

2004/Makefile

@@ -50,7 +50,7 @@
 # tool locations
 ################
 #
-SHELL= bash
+SHELL= /bin/bash
 MAKE= make