app-crypt/veracrypt: fix the linking against wxGTK[X] when USE='-X'

app-crypt/veracrypt: fix the linking against wxGTK[X] when USE='-X'

veracrypt[-X] fails to properly link against wxGTK[X] only using the
'base' library because the wx's setup.h implicitly defines
wxUSE_GUI=1, which then changes the definition of various structures,
requiring symbols from the core library as well.

For example, wxwidgets include/wx/app.h defines:

  #if wxUSE_GUI

  class WXDLLIMPEXP_CORE wxAppBase : public wxAppConsole
  ...
  [snip]
  ...
  #else // !GUI

  // wxApp is defined in core and we cannot define another one in wxBase,
  // so use the preprocessor to allow using wxApp in console programs too
  #define wxApp wxAppConsole

  #endif // GUI/!GUI

To fix this, pass '-DwxUSE_GUI=0' when compiling veracrypt[-X].

Closes: https://bugs.gentoo.org/605018
Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Göktürk Yüksek <gokturk@gentoo.org>