Conditional building of Xpm (and possibly others?)

[Kean Johnston]

> The new libXaw requires libXpm. Do you want to use an existing libXpm or 
>  disable it entierly ? In the latter case, you need to also remove the 
> code using Xpm in Xaw or disable Xaw.
No I was thinking specifically of the case where the system already
provides Xpm, not for removing it altogether. The change I've made
treats it much the same was as libraries like expat, freetype etc
are treated: give the OS *.cf the opportunity to tell the system
that we already have the library and don't need to build the one
in the Xorg tree.

Kean