glew: Don't hard code Win32 platform on Windows

Change-Id: I753602314f0ed3822bce489e9e427a1de77d2431
Reviewed-on: https://gerrit.libreoffice.org/13230
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>