WIN32 is not necessarily defined _WIN32 should be #24
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Typo or intentional?
_WIN32 is described in https://msdn.microsoft.com/en-us/library/b0084kay(v=vs.140).aspx for all editions WIN32 is not defined for any. Thus the reason to not change this, would be to somehow take into account windows c++ compilers that does not define this flag by default. However, on my combination of cmake flags WIN32 were not defined at compile-time.
(On Windows 8.1 with Visual studio v140, v120 and the Intel C++ Compiler 17.0 - Using ExternalProject_Add almost the same way Slicer do, but without the VTK dependency)