Provide expanded prefix and exec_prefix versions (named

expanded_SOMETHING).

m4/set-prefix.m4

@@ -4,4 +4,8 @@ AC_DEFUN(AC_SET_PREFIX,
 [test "x$prefix" = xNONE && prefix=$ac_default_prefix
 # Let make expand exec_prefix.
 test "x$exec_prefix" = xNONE && exec_prefix='${prefix}'
+expanded_prefix=`eval echo ${prefix}`
+AC_SUBST(expanded_prefix)
+expanded_exec_prefix=`eval echo ${exec_prefix}`
+AC_SUBST(expanded_exec_prefix)
 ])