Revert "Disable stack protection for mingw win32 builds in 2.4"

This reverts commit 298f1ab57000afd7acb8c0ddbddca8a1fc6cd92a.

We're shipping libssp-0.dll now with mingw builds. We would
have enough time to test this for 2.5.0 now anyway, but we
also will most likely switch to MSVC for that release.

src/CMakeLists.txt

@@ -13,7 +13,7 @@ if(NOT TOKEN_AUTH_ONLY)
     find_package(Qt5Keychain REQUIRED)
 endif()
 
-if(NOT WIN32)
+if(NOT MSVC)
   if(NOT (CMAKE_SYSTEM_PROCESSOR MATCHES "^(alpha|parisc|hppa)"))
     if((CMAKE_CXX_COMPILER_ID MATCHES "GNU") AND (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9))
       set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fstack-protector --param=ssp-buffer-size=4")