Made the CMake config more submodule friendly.

CMakeLists.txt

@@ -257,7 +257,7 @@ ELSEIF(MSVC)
   IF(MSVC12)
     ADD_COMPILE_OPTIONS(/wd4351)
   ENDIF()
-  SET(CMAKE_CXX_FLAGS_DEBUG "/D_DEBUG /MDd /Zi /Od")
+  SET(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /D_DEBUG /Zi /Od")
 ELSEIF ( "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
   IF(NOT HUNTER_ENABLED)
     SET(CMAKE_CXX_FLAGS "-fPIC -std=c++11 ${CMAKE_CXX_FLAGS}")