Platform specific knowledge: WIN32_FILE and STD_FILE are mutually exc…

…lusive options

Therefore, when we define `WIN32_FILE_SUPPORT` we must also define `NO_STD_FILE_SUPPORT`
Until now, this knowledge was in build.win*/common/Makefile.*
(and a legacy MSVC project in plaftorms/win32/misc/Squeak.dsp - on pourrait réduire la voilure !!!)
Since it is easy to forget the second define, lets extend the knowledge in the sqPlatformSpecific.h

platforms/win32/vm/sqPlatformSpecific.h

@@ -30,6 +30,7 @@
 
 #ifdef WIN32_FILE_SUPPORT
 
+#define NO_STD_FILE_SUPPORT /* STD_FILE or WIN32_FILE are mutually exclusive options */
 #undef sqImageFile
 #undef sqImageFileClose
 #undef sqImageFileOpen