Add platform-specific fixes file

Create a new file, platform.h, for platform-specific hacks

for MSVC, this includes defining strncasecmp to _stricmp and
strdup to _strdup, among other things like defining missing
stat macros

Change some things not supported in MSVC, like _Static_assert,
to their counterparts (in this case, static_assert)

Replace usage of VLAs with malloc and free

Update getopt_long and use the getopt implementation from musl
on Windows.

Use comments to show which functions from platform.h are being used

src/asm/fstack.c

@@ -19,7 +19,6 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
-#include <unistd.h>
 
 #include "asm/fstack.h"
 #include "asm/lexer.h"
@@ -30,6 +29,7 @@
 
 #include "extern/err.h"
 
+#include "platform.h" // S_ISDIR (stat macro)
 #include "types.h"
 
 static struct sContext *pFileStack;