Make improvements

- You can now run `make -j8 toolchain` on Windows
- You can now run `make -j` on MacOS ARM64 and BSD OSes
- You can now use our Emacs dev environment on MacOS/Windows
- Fix bug where the x16 register was being corrupted by --ftrace
- The programs under build/bootstrap/ are updated as fat binaries
- The Makefile now explains how to download cosmocc-0.0.12 toolchain
- The build scripts under bin/ now support "cosmo" branded toolchains
- stat() now goes faster on Windows (shaves 100ms off `make` latency)
- Code cleanup and added review on the Windows signal checking code
- posix_spawnattr_setrlimit() now works around MacOS ARM64 bugs
- Landlock Make now favors posix_spawn() on non-Linux/OpenBSD
- posix_spawn() now has better --strace logging on Windows
- fstatat() can now avoid EACCES in more cases on Windows
- fchmod() can now change the readonly bit on Windows

third_party/make/filename.h

@@ -29,23 +29,13 @@ extern "C" {
                         it may be concatenated to a directory pathname.
    IS_PATH_WITH_DIR(P)  tests whether P contains a directory specification.
  */
-#if defined _WIN32 || defined __CYGWIN__ || defined __EMX__ || defined __DJGPP__
-  /* Native Windows, Cygwin, OS/2, DOS */
-# define ISSLASH(C) ((C) == '/' || (C) == '\\')
-# define HAS_DEVICE(P) \
-    ((((P)[0] >= 'A' && (P)[0] <= 'Z') || ((P)[0] >= 'a' && (P)[0] <= 'z')) \
-     && (P)[1] == ':')
-# define IS_ABSOLUTE_PATH(P) (ISSLASH ((P)[0]) || HAS_DEVICE (P))
-# define IS_PATH_WITH_DIR(P) \
-    (strchr (P, '/') != NULL || strchr (P, '\\') != NULL || HAS_DEVICE (P))
-# define FILE_SYSTEM_PREFIX_LEN(P) (HAS_DEVICE (P) ? 2 : 0)
-#else
-  /* Unix */
 # define ISSLASH(C) ((C) == '/')
-# define IS_ABSOLUTE_PATH(P) ISSLASH ((P)[0])
-# define IS_PATH_WITH_DIR(P) (strchr (P, '/') != NULL)
-# define FILE_SYSTEM_PREFIX_LEN(P) 0
-#endif
+# define HAS_DEVICE(Filename) ((void) (Filename), 0)
+# define FILE_SYSTEM_PREFIX_LEN(Filename) ((void) (Filename), 0)
+# define FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE 0
+# define IS_ABSOLUTE_FILE_NAME(Filename) ISSLASH ((Filename)[0])
+# define IS_RELATIVE_FILE_NAME(Filename) (! ISSLASH ((Filename)[0]))
+# define IS_FILE_NAME_WITH_DIR(Filename) (strchr ((Filename), '/') != NULL)
 
 
 #ifdef __cplusplus