This reverts commit 94945a719a.
It was too intrusive and can be handled by the 'onefile' script
by compiling isaac64.c before any source file that includes hack.h.
isaac64.c includes <math.h>. yn() is a non-STDC math function in
<math.h> and that conflicts with nethack's yn() macro or vice versa.
If other source files begin using <math.h> this will probably need
to be handled differently.
This is the version from the Comprehensive C Archive Network, licensed
under the CC0 "No Rights Reserved" Creative Common License.
http://ccodearchive.net/info/isaac.html
