Adds #defines that ensures that DllMain is only compiled in when the thread-safe shared library is built on Windows.

Fixes HDFFV-8837.

Tested on 64-bit Windows 7 with VS2012.

git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@25274 dab4d1a6-ed17-0410-a064-d1ae371a2980