build: Remove support for cross-building from Cygwin and SunOS.

These have been broken for a long time. Some Cygwin changes that
are relevant on MinGW are kept here, but users on Windows who
want to build Haiku should probably use WSL at this point.

However, now that we are using relative paths and don't need
to worry about drive path kludges, it's actually possible to
get some host tools built on MinGW. Changes for that coming.

headers/build/BeOSBuildCompatibility.h

@@ -1,20 +1,12 @@
 #ifndef BEOS_BUILD_COMPATIBILITY_H
 #define BEOS_BUILD_COMPATIBILITY_H
 
-#if defined(HAIKU_HOST_PLATFORM_CYGWIN)
-#	ifndef __addr_t_defined
-#		define __addr_t_defined
-#	endif
-#endif
-
-// DEFFILEMODE is not available on Cygwin, SunOS and when building with musl c
-#if defined(HAIKU_HOST_PLATFORM_CYGWIN) || defined(HAIKU_HOST_PLATFORM_SUNOS) \
-	|| !defined(DEFFILEMODE)
+// DEFFILEMODE is not available on MinGW and on platforms with MUSL
 #ifndef DEFFILEMODE
 #define DEFFILEMODE (S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH)
 #endif
 
-// There's no ALLPERMS when building with musl c
+// There's no ALLPERMS on platforms with MUSL
 #ifndef ALLPERMS
 #	define ALLPERMS (S_ISUID|S_ISGID|S_ISVTX|S_IRWXU|S_IRWXG|S_IRWXO)
 #endif
@@ -23,16 +15,6 @@
 #define	S_IUMSK 07777
 #endif
 
-#include <ctype.h>
-#endif
-
-#ifdef HAIKU_HOST_PLATFORM_SUNOS
-#	include <limits.h>
-#	ifndef NAME_MAX
-#		define NAME_MAX	MAXNAMELEN
-#	endif
-#endif
-
 typedef unsigned long	haiku_build_addr_t;
 #define addr_t			haiku_build_addr_t
 
@@ -62,7 +44,7 @@ extern size_t	strlcat(char* dest, const char* source, size_t length);
 extern size_t	strnlen(const char* string, size_t length);
 #endif
 
-#if defined(HAIKU_HOST_PLATFORM_CYGWIN) || defined(HAIKU_HOST_PLATFORM_SUNOS)
+#if defined(HAIKU_HOST_PLATFORM_MINGW)
 extern char*	stpcpy(char* dest, const char* src);
 extern char*	strcasestr(const char* s, const char* find);
 #endif