FindBoost: Add -vc120 mangling for VS 12

Modules/FindBoost.cmake

@@ -349,6 +349,8 @@ function(_Boost_GUESS_COMPILER_PREFIX _ret)
     else()
       set (_boost_COMPILER "-il")
     endif()
+  elseif (MSVC12)
+    set(_boost_COMPILER "-vc120")
   elseif (MSVC11)
     set(_boost_COMPILER "-vc110")
   elseif (MSVC10)