[vtkusers] MinGW visibility attribute not supported warning

[Sascha Zelzer]

Hi,

when building VTK with MinGW, a lot of visibility warnings ("visibility 
attribute not supported in this configuration") are displayed. This is 
due to

set(VTK_ABI_CXX_FLAGS "-fvisibility=hidden -fvisibility-inlines-hidden")

in line 43 of vtkCompilerExtras.cmake. Although MinGW is a gcc variant, 
it is not necessary to set the visibility flags if you use 
__declspec(export) attributes and don't use --export-all-symbols. See 
for example 
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/gnu-linker/win32.html

I attached a patch which would fix this issue by not adding the 
visibility flags if the compiler is MinGW. Any feedback is appreciated.

Thanks,
Sascha