make it less likely to have people get confused by Eigen3_DIR

cmake/CMakeLists.txt

@@ -538,6 +538,13 @@ if(PKG_USER-NETCDF)
 endif()
 
 if(PKG_USER-SMD)
+  # hack alert!
+  # something defines the Eigen3_DIR variable, but it has no effect
+  # yet shows up in the TUI/GUI display as "not found".
+  # and due to not being marked as advanced, it can mislead people
+  # as the EIGEN3_INCLUDE_DIR variable which does have an effect
+  # is marked as advanced. Tested on Fedora28 with Eigen3-3.3.5 rpm.
+  mark_as_advanced(Eigen3_DIR)
   option(DOWNLOAD_EIGEN3 "Download Eigen3 (instead of using the system's one)" OFF)
   if(DOWNLOAD_EIGEN3)
     include(ExternalProject)