[lmi] Difficulty building with "dll" attributes and new wx

[Greg Chicares]

To reproduce, either do this:
  ./nychthemeral_test.sh
with current head (commit 6977670839d9ee, which upgraded wx),
or manually upgrade wx as that commit prescribes and do this:
  make all build_type=so_test USE_SO_ATTRIBUTES=1
This link command fails:

i686-w64-mingw32-g++ -o skeleton.dll -shared convenience.o exception.o 
path_posix_windows.o operations_posix_windows.o xml_xslt_wrapp.o about_dialog.o 
alert_wx.o census_document.o census_view.o database_document.o database_view.o 
database_view_editor.o default_view.o docmanager_ex.o file_command_wx.o 
gpt_document.o gpt_view.o group_quote_pdf_gen_wx.o icon_monger.o 
illustration_document.o illustration_view.o input_sequence_entry.o 
main_common.o mec_document.o mec_view.o msw_workarounds.o multidimgrid_any.o 
multidimgrid_tools.o mvc_controller.o mvc_view.o pdf_command_wx.o 
pdf_writer_wx.o policy_document.o policy_view.o preferences_view.o 
previewframe_ex.o product_editor.o progress_meter_wx.o rounding_document.o 
rounding_view.o rounding_view_editor.o single_choice_popup_menu.o skeleton.o 
system_command_wx.o text_doc.o text_view.o tier_document.o tier_view.o 
tier_view_editor.o transferor.o view_ex.o wx_checks.o wx_table_generator.o 
wx_utility.o liblmi.dll wx_new.dll -L . -L 
/opt/lmi/local/gcc_i686-w64-mingw32/lib -L 
/opt/lmi/local/gcc_i686-w64-mingw32/bin -lwxcode_mswu_pdfdoc-3.1  -L 
/opt/lmi/local/gcc_i686-w64-mingw32/lib -L 
/opt/lmi/local/gcc_i686-w64-mingw32/lib  -lwx_mswu-3.1-i686-w64-mingw32 
-mwindows    -lexslt -lxslt -lxml2      -Wl,-Map,skeleton.dll.map  
-Wl,--disable-auto-import -static-libstdc++

producing lots of messages like these:

/usr/bin/i686-w64-mingw32-ld: about_dialog.o: in function `ZN7wxPanelC4Ev':
/opt/lmi/local/include/wx-3.1/wx/msw/panel.h:22: undefined reference to `vtable 
for wxPanel'
/usr/bin/i686-w64-mingw32-ld: about_dialog.o: in function 
`ZN16wxScrolledWindowC4Ev':
/opt/lmi/local/include/wx-3.1/wx/scrolwin.h:497: undefined reference to `vtable 
for wxScrolledWindow'

I count undefined references to vtables for 142 different things
(filtered with 'sort -u'); all are in wx (none point elsewhere).

I'm using gcc-10:
  $ i686-w64-mingw32-gcc --version
  i686-w64-mingw32-gcc (GCC) 10-win32 20200525
but 'nychthemeral_test.sh' ran cleanly with that version (as well as
with gcc-8.3) before this wx upgrade. I could test it with gcc-8.3 too,
but that's inconvenient at the moment.