If Windows and shared, force .dll extension.

CMakeLists.txt

@@ -42,6 +42,9 @@ if (WINDOWS)
     ADD_DEFINITIONS(-D WINDOWS=1)
     # -m64: Build a 64 bit library
     add_compile_options(-m64)
+    if (SHARED)
+        set_target_properties(Arbutils PROPERTIES SUFFIX ".dll")
+    endif()
 endif (WINDOWS)
 
 if (CMAKE_BUILD_TYPE MATCHES "Debug")