#196 Fix failed compilation on Fedora due to missing header error

[Trung Le]

Fedora's motif-devel ships private (P-suffix) headers — e.g.
Xm/DisplayP.h — that begin with:

  #ifdef HAVE_CONFIG_H
  #include <config.h>
  #endif

CDE compiles with -DHAVE_CONFIG_H (it owns include/cde_config.h via
AC_CONFIG_HEADERS), so when DtWidget pulls in Xm/DisplayP.h the Motif
header tries to include a config.h that doesn't exist on the search
path, breaking the build:

  /usr/include/Xm/DisplayP.h:27:10: fatal error: config.h: No such
  file or directory