[ale] Looking for X11 SDK to build Source-Navigator 5.1.4

[johnmills at speakeasy.net (John Mills)]

ALErs -

I am trying to build S-N from the tarball on a Slack-9.1 installation, but
the build dies on missing files: 'libX.h', 'Xproto.h', 'X.h' and friends.
SourceForge mail suggests there is a set of needed files in an 'X11 SDK',
and that S-N's 'configure' doesn't notice their absence (but 'make' does
&8-).

I come up dry looking for such a SDK, nor do I know what dependencies _it_ 
might have.

TIA for any pointers.

 - John Mills
   john.m.mills at alum.mit.edu