(jm_AC_DOS): Rewrite (though it's still a stub) to work better with autoheader.

m4/dos.m4

@@ -1,4 +1,4 @@
-# serial 1
+# serial 2
 
 # Define some macros required for proper operation of code in lib/*.c
 # on MSDOS/Windows systems.
@@ -11,17 +11,27 @@ AC_DEFUN(jm_AC_DOS,
     # like this:
     #if defined _WIN32 || defined __WIN32__ || defined __MSDOS__
 
-    ac_fspl_def="((Filename)[0] && (Filename)[1] == ':' ? 2 : 0)"
-    ac_fspl_def=0
-    AC_DEFINE_UNQUOTED([FILESYSTEM_PREFIX_LEN(Filename)], $ac_fspl_def,
-      [On systems for which file names may have a so-called `drive letter'
-       prefix, define this to compute the length of that prefix, including
-       the colon.  Otherwise, define it to zero.])
+    AH_VERBATIM(FILESYSTEM_PREFIX_LEN,
+    [#if FILESYSTEM_ACCEPTS_DRIVE_LETTER_PREFIX
+# define FILESYSTEM_PREFIX_LEN(Filename) \
+  ((Filename)[0] && (Filename)[1] == ':' ? 2 : 0)
+else
+# define FILESYSTEM_PREFIX_LEN(Filename) 0
+#endif])
 
-    ac_isslash_def="((C) == '/' || (C) == '\\')"
-    ac_isslash_def="((C) == '/')"
-    AC_DEFINE_UNQUOTED([ISSLASH(C)], $ac_isslash_def,
-      [Define to return nonzero for any character that may serve as
-       a file name component separator.  On POSIX systems, it is the
-       slash character.  Some other systems also accept backslash.])
+    AC_DEFINE([FILESYSTEM_ACCEPTS_DRIVE_LETTER_PREFIX],
+      [Define on systems for which file names may have a so-called
+       `drive letter' prefix, define this to compute the length of that
+       prefix, including the colon.])
+
+    AH_VERBATIM(ISSLASH,
+    [#if FILESYSTEM_BACKSLASH_IS_FILE_NAME_SEPARATOR
+# define ISSLASH(C) ((C) == '/' || (C) == '\\\\')
+#else
+# define ISSLASH(C) ((C) == '/')
+#endif])
+
+    AC_DEFINE([FILESYSTEM_BACKSLASH_IS_FILE_NAME_SEPARATOR],
+      [Define if the backslash character may also serve as a file name
+       component separator.])
   ])