make the post-build copy instruction valid for multi-configuration build system

with msvc, the CMAKE_BUILD_TYPE is not taken into account from CMake.
MSVC is multi-configuration and we can change the config from inside msvc.
This new cmake code allows to use this multi-configuration system and
generate automatically the right post-build instruction

cmake/ACGCommon.cmake

@@ -791,15 +791,9 @@ function (acg_add_library _target _libtype)
     endif ()
   endif ()
 
-  if( ${CMAKE_BUILD_TYPE} STREQUAL Debug )
-    set ( postfix ${CMAKE_DEBUG_POSTFIX} )
-  else ()
-    set ( postfix "" )
-  endif ()
-
+  set( postfix $<IF:$<CONFIG:Debug>,${CMAKE_DEBUG_POSTFIX},"">)
   set( fullname ${_target}${postfix} )
 
-
   if (WIN32)
     # copy exe file to "Build" directory
     # Visual studio will create this file in a subdirectory so we can't use