Previously we would try to build a couple files which can only be
built when SLIVR is available, causing build failures.  This commit
makes compilation of some Volume plot files configuration-dependent,
allowing non-SLIVR-enabled builds to succeed.

* configure.in, include/make-variables.in: AC_SUBST two new variables,
  SLIVR_GUI_IMPL and SLIVR_GUI_HDR, which contain the names of the SLIVR
  GUI files to be compiled (empty when SLIVR is not present).
* Source files
    * Use forward declarations and HAVE_LIBSLIVR to avoid issues when
      SLIVR is not available.
    * Reorder includes so that SLIVR is included before Qt, avoiding a
      conflict on the `emit' symbol.
* Volume.xml: Use the AC_SUBST'd variables instead of the filenames.

git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@5216 18c085ea-50e0-402c-830e-de6fd14e8384