ac_subst_prefix_subpaths

look at varname and detect the subpath that it contains relative to $prefix/$exec_prefix

If the path is indeed relative to $prefix/$exec_prefix, then a single "./" (dotslash) is prepended, otherwise it can be seen as an absolute path that can not be moved, which you possibly do for "/etc" files, or even those ending up in "/lib/modules" or "/winnt/system".

this macro is not very intelligent, it's just a first try in this direction. It does currently just look into the current patterns, and replaces a ${prefix} with a simple dot. Amazingly, it works quite well for most packages.

correct usage: in configure ac
    AC_DEFINE_DIR([EPREFIX], [exec_prefix], [--exec-prefix or default])
    AC_DEFINE_SUB_PATH([PATH_LIBDIR], [libdir], [--bindir subdir])
    AC_DEFINE_UNQUOTED([PACKAGE],"$PACKAGE", [Name of package])
in "C"
    static const char _libdir[] = PATH_LIBDIR; /* configure default */
    char* libdir;
    char* eprefix = getenv (PACKAGE "DIR");
    if (! eprefix) eprefix = EPREFIX; /* default */
    if (*_libdir != '.') libdir = strdup(_libdir);
    else {
       libdir = malloc(strlen(eprefix) + strlen(_libdir) + 2);
       strcpy(libdir, eprefix);
       strcat(libdir, PATH_DELIMITER_STRING);
       strcat(libdir, _libdir);
    }
    ...
    free (libdir);
          
    
AC_DEFINE_SUB_PATHS(varnames)

look for the given various install-paths that largely depend on either ${prefix} or ${exec_prefix}. Just cut out the prefix and ac_define the value. The value is uppercased and PATH_ prepended ie. ac_define_sub_paths(bindir libdir pkgdatadir) will create the defines PATH_BINDIR PATH_LIBDIR PATH_PKGDATADIR - see posix' include/paths.h that creates _PATH_DEV and friends.

@author: Guido Draheim <guidod@gmx.de>

AC_DEFUN([AC_DEFINE_SUB_PATH],
[dnl
  test "_$prefix" = _NONE && prefix="$ac_default_prefix"
  test "_$exec_prefix" = _NONE && exec_prefix='${prefix}'
  P=`echo ifelse( $2, , [$]$1, [$]$2) | sed -e 's:^\${[a-z_]*prefix}:.:'`
  ifelse ($3, , 
    AC_DEFINE($1, $P, [sub path $2]),
    AC_DEFINE($1, $P, $3))
])

AC_DEFUN([AC_DEFINE_SUB_PATHS],
[dnl
  test "_$prefix" = _NONE && prefix="$ac_default_prefix"
  test "_$exec_prefix" = _NONE && exec_prefix='${prefix}'
  for i in $1 ; do
  P=`echo \$$i | sed -e 's:^\${[a-z_]*prefix}:.:'`
  V=`echo path_$i | sed -e 'y:abcdefghijklmnopqrstuvwxyz:ABCDEFGHIJKLMNOPQRSTUVWXYZ:'`
    AC_DEFINE($V, $P, [sub path $i]),
])