CMake files of VTK 6.0 do not handle newer GCC output correctly and thus fail with:
CMake Error at /usr/lib/cmake/vtk-6.0/GenerateExportHeader.cmake:177 (if):
if given arguments:
"gcc-8 (Ubuntu 8.1.0-5ubuntu1~14.04) 8.1.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software" " see the source for copying conditions. There is
NO
warranty" " not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE." "VERSION_LESS" "4.2"
Unknown arguments specified
Call Stack (most recent call first):
/usr/lib/cmake/vtk-6.0/GenerateExportHeader.cmake:352 (_test_compiler_hidden_visibility)
...
So we just drop those bogus GNU checks in VTK's GenerateExportHeader.cmake
which is exactly what newer VTK versions do.