Pull request from bernhardreiter: NetHack.ad has a comment about
needing to use an external tool such as XV or PBMplus rather than
the NetHack.double_tile_size resource if nethack is built with the
USE_XPM configuration. Add some more detail since using 'hints' when
setting up the Makefiles can define that behind the builder's back.
The extra detail won't be useful to players who obtain prebuilt
binaries that incorporate the X11 interface. The comment in config.h
(see preceding pull request) won't be either, and maybe should be
moved to NetHack.ad where such users will be able to see it.
Fixes#1114.
8dfcdfe753