Add Boost_USE_STATIC_LIBS parameter for Windows

If this parameter is not set in windows, cmake expects
boost libraries to be named "boost_*.lib", while they are
actually "libboost_*.lib"

In findBoost.cmake (728):

  # Setting some more suffixes for the library
  set (Boost_LIB_PREFIX "")
  if (WIN32 AND Boost_USE_STATIC_LIBS AND NOT CYGWIN)
    set(Boost_LIB_PREFIX "lib")
  endif()

CMakeLists.txt

@@ -168,6 +168,7 @@ set(OPENMW_LIBS_HEADER)
 
 # Platform specific
 if (WIN32)
+    set(Boost_USE_STATIC_LIBS   ON)
     set(PLATFORM_INCLUDE_DIR "platform")
     add_definitions(-DBOOST_ALL_NO_LIB)
 else (WIN32)