COMP: OpenJPEG #ifdef WIN32 not reliable ITK-3101.

It appears that in Modules/ThirdParty/OpenJEPEG/src/openjpeg/opj_malloc.h in
line 67 the '#ifdef WIN32' results in 0 on my system, Vista x64, VS9. As a
result the linker expects posix_memalign. This causes multiple build errors.

Suggested-by: Constantine Zakkaroff <mnkz@leeds.ac.uk>

Prefer to use the _WIN32 preprocessor definition defined intrinsically by the
compiler as opposed to "WIN32" defined by the SDKs.

Change-Id: Idd94085cdad4fccd302deef57975c305d3c85d22
Merge branch 'WIN32def' into release
Merge branch 'release'