CMake: fix Windows detection

Fixes #201.

CMakeLists.txt

@@ -34,7 +34,7 @@ option(BUILD_SHARED_LIBS "Build as shared library" ON)
 if (NOT CMAKE_CONFIGURATION_TYPES AND NOT CMAKE_BUILD_TYPE)
   set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS "Debug;Release;RelWithDebInfo;MinSizeRel")
 endif()
-if(WINDOWS)
+if(WIN32)
   set(CMAKE_DEBUG_POSTFIX d CACHE STRING "Postfix for debug version of library")
 endif()