use "-O2 -DNDEBUG" in release by default in the case of MinGW

CMakeLists.txt

@@ -41,9 +41,12 @@ set(CMAKE_SHARED_LINKER_FLAGS_MINSIZEREL "" CACHE INTERNAL "" FORCE)
 set(CMAKE_SHARED_LINKER_FLAGS_RELWITHDEBINFO "" CACHE INTERNAL "" FORCE)
 set(CMAKE_VERBOSE OFF CACHE BOOL "Verbose mode")
 
-if (MINGW)
+
+if (WIN32)
+    if(${CMAKE_GENERATOR} MATCHES "(MinGW)|(MSYS)")
         set(CMAKE_CXX_FLAGS_RELEASE "-O2 -DNDEBUG" CACHE STRING "")
     endif()
+endif()
 
 if(CMAKE_VERBOSE)
     set(CMAKE_VERBOSE_MAKEFILE 1)