CMake: correctly detect 64bit for MinGW compilers

CMAKE_CL_64 is only defined when using the 64 bit cl compiler from Microsoft.

build/cmake/init.cmake

@@ -59,7 +59,7 @@ if(WIN32)
         message(FATAL_ERROR "Unknown WIN32 compiler type")
     endif()
 
-    if(CMAKE_CL_64)
+    if(CMAKE_SIZEOF_VOID_P EQUAL 8)
         set(wxARCH_SUFFIX "_x64")
     endif()
 else()

build/cmake/main.cmake

@@ -53,7 +53,7 @@ if(MSVC OR MINGW OR CYGWIN)
     else()
         set(wxREQUIRED_OS_DESC "Windows Vista / Windows Server 2008")
     endif()
-    if(CMAKE_CL_64)
+    if(CMAKE_SIZEOF_VOID_P EQUAL 8)
         wx_string_append(wxREQUIRED_OS_DESC " (x64 Edition)")
     endif()
 elseif(APPLE AND NOT IPHONE)