AH_VERBATIM isn't really verbatim with backslashes (autoconf 2.49c)

[Paul Eggert]

Given the following configure.ac file:

  AC_INIT(configure.ac)
  AC_CONFIG_HEADER(config.h)
  AH_VERBATIM([BACKSLASH], [#define BACKSLASH '\\'])

The command "autoheader" generates a config.h.in file like this:

  /* config.h.in.  Generated automatically from configure.ac by autoheader.  */

  #define BACKSLASH '\'

Apparently AH_VERBATIM isn't really verbatim: it turned two
backslashes into one.  This isn't documented anywhere that I can see;
is it intended?

I ran into this problem when trying to figure out why fileutils 4.0.38
uses AH_VERBATIM with "too many" backslashes.  fileutils 4.0.38
contains code that looks like this:

    AH_VERBATIM(ISSLASH,
    [#if FILESYSTEM_BACKSLASH_IS_FILE_NAME_SEPARATOR
# define ISSLASH(C) ((C) == '/' || (C) == '\\\\')
#else
# define ISSLASH(C) ((C) == '/')
#endif])

and that \\\\ works around the fact that autoheader turns two
backslashes into one.