smtk: pass the install dir to SMTK

This lets SMTK know that it is using the superbuild.

projects/smtk.cmake

@@ -53,6 +53,11 @@ superbuild_add_project(smtk
     # Windows needs help finding the ZeroMQ from the install.
     -DSMTK_RELOCATABLE_INSTALL:BOOL=OFF
 
+    # DLL path loading help for the build tree.
+    -D_smtk_superbuild_prefix:PATH=<INSTALL_DIR>
+    # Not needed for the install tree.
+    -DSMTK_UNIFIED_INSTALL_TREE:BOOL=ON
+
     ${smtk_extra_cmake_args}
     -DBUILD_SHARED_LIBS:BOOL=ON
     -DCMAKE_INSTALL_LIBDIR:STRING=lib