Fix CMake errors of VTK 6.0 with newer GCC revisions

README.md

Fix CMake errors of VTK 6.0 with newer GCC revisions

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.

vtk-6.0_GenerateExportHeader.cmake.patch

--- a/usr/lib/cmake/vtk-6.0/GenerateExportHeader.cmake	2013-06-12 21:47:10.000000000 +0200
+++ b/usr/lib/cmake/vtk-6.0/GenerateExportHeader.cmake	2019-07-05 11:03:41.237221206 +0200
@@ -163,20 +163,7 @@
 macro(_test_compiler_hidden_visibility)
 
   if(CMAKE_COMPILER_IS_GNUCXX)
-    exec_program(${CMAKE_C_COMPILER} ARGS --version
-      OUTPUT_VARIABLE _gcc_version_info)
-    string(REGEX MATCH "[345]\\.[0-9]\\.[0-9]"
-      _gcc_version "${_gcc_version_info}")
-    # gcc on mac just reports: "gcc (GCC) 3.3 20030304 ..." without the
-    # patch level, handle this here:
-    if(NOT _gcc_version)
-      string(REGEX REPLACE ".*\\(GCC\\).* ([34]\\.[0-9]) .*" "\\1.0"
-        _gcc_version "${_gcc_version_info}")
-    endif()
-
-    if(${_gcc_version} VERSION_LESS "4.2")
-      set(GCC_TOO_OLD TRUE)
-    endif()
+    set(GCC_TOO_OLD FALSE)
   endif()
 
   if(CMAKE_CXX_COMPILER_ID MATCHES Intel)