message: cmake: help MSVC find M_PI
diff: @@ -3,7 +3,7 @@ if(MINGW)
  endif()
  if(MSVC)
  - add_definitions(-DWINDOWS)
  + add_definitions(-DWINDOWS -D_USE_MATH_DEFINES)
  set(warnings "-WX -W2")
  set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
  