linking: default to not making warnings fatal errors

Toolchains tend to add new warnings as time goes on, so in the spirit of
keeping VTK as future-proof as possible, don't turn them into warnings
by default. Testing machines and developers can certainly enable it
though.

    http://public.kitware.com/pipermail/vtk-developers/2015-July/032233.html

CMake/vtkCompilerExtras.cmake

@@ -14,12 +14,17 @@ if(CMAKE_COMPILER_IS_GNUCXX)
 
   # If we are compiling on Linux then set some extra linker flags too
   if(CMAKE_SYSTEM_NAME MATCHES Linux)
+    option(VTK_LINKER_FATAL_WARNINGS "Specify if linker warnings must be considered as errors." OFF)
+    mark_as_advanced(VTK_LINKER_FATAL_WARNINGS)
+    if(VTK_LINKER_FATAL_WARNINGS)
+      set(VTK_EXTRA_SHARED_LINKER_FLAGS "-Wl,--fatal-warnings")
+    endif()
     set(CMAKE_SHARED_LINKER_FLAGS
-      "-Wl,--fatal-warnings -Wl,--no-undefined -lc ${CMAKE_SHARED_LINKER_FLAGS}")
+      "${VTK_EXTRA_SHARED_LINKER_FLAGS} -Wl,--no-undefined -lc ${CMAKE_SHARED_LINKER_FLAGS}")
     set(CMAKE_MODULE_LINKER_FLAGS
-      "-Wl,--fatal-warnings -Wl,--no-undefined -lc ${CMAKE_SHARED_LINKER_FLAGS}")
+      "${VTK_EXTRA_SHARED_LINKER_FLAGS} -Wl,--no-undefined -lc ${CMAKE_SHARED_LINKER_FLAGS}")
     set (CMAKE_EXE_LINKER_FLAGS
-      "-Wl,--fatal-warnings -Wl,--no-undefined -lc ${CMAKE_SHARED_LINKER_FLAGS}")
+      "${VTK_EXTRA_SHARED_LINKER_FLAGS} -Wl,--no-undefined -lc ${CMAKE_SHARED_LINKER_FLAGS}")
   endif()
 
   # Now check if we can use visibility to selectively export symbols