No check for xpm.h in gnome-libs configure

[Erik Walthinsen <omega temple-baptist com>]

I just wasted an hour with a failed build of gnome-libs-1.0.55 on a
NetWinder, because xpm-devel wasn't installed.  There is no check in
configure.in for this file, so it fails far into the build.  I looked at
the latest in CVS, and there is still no check.  Either that or it's
non-obvious.  If there isn't one there, one should probably be added.

      Erik Walthinsen <omega temple-baptist com> - System Administrator
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