Set DESTDIR if relative loading

When relative loading, `prefix` makes no sense actually.  Use the
given (or default) path as `DESTDIR` instead.

This change affects only when the relative loading is enabled and the
destdir is not given, and does not change the final installation path,
but makes the configuration options simpler a little.

Author: nobu

configure.ac

@@ -4753,6 +4753,11 @@ AC_ARG_WITH(destdir,
 	    [DESTDIR="$withval"])
 AC_SUBST(DESTDIR)
 
+AS_IF([test "x$load_relative:$DESTDIR" = xyes:], [
+    AS_IF([test "x$prefix" = xNONE], [DESTDIR="$ac_default_prefix"], [DESTDIR="$prefix"])
+    prefix=/.
+])
+
 AC_OUTPUT
 }