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/asmy.y

@@ -15,7 +15,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <strings.h>
 
 #include "asm/asm.h"
 #include "asm/charmap.h"
@@ -34,6 +33,7 @@
 #include "extern/utf8decoder.h"
 
 #include "linkdefs.h"
+#include "platform.h" // strncasecmp, strdup
 
 uint32_t nListCountEmpty;
 char *tzNewMacro;

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;

src/asm/lexer.c

@@ -13,7 +13,6 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
-#include <strings.h>
 
 #include "asm/asm.h"
 #include "asm/fstack.h"
@@ -27,6 +26,7 @@
 #include "extern/err.h"
 
 #include "asmy.h"
+#include "platform.h" // strncasecmp, strdup
 
 struct sLexString {
 	char *tzName;

src/asm/output.c

@@ -30,6 +30,7 @@
 #include "extern/err.h"
 
 #include "linkdefs.h"
+#include "platform.h" // strdup
 
 struct Patch {
 	char tzFilename[_MAX_PATH + 1];

src/asm/section.c

@@ -14,6 +14,7 @@
 #include "asm/warning.h"
 
 #include "extern/err.h"
+#include "platform.h" // strdup
 
 struct SectionStackEntry {
 	struct Section *pSection;