Don't build readline/libreadline.a, when --with-system-readline is supplied

Don't build readline/libreadline.a, when --with-system-readline is supplied

https://sourceware.org/bugzilla/show_bug.cgi?id=18632

The bundled libreadline is always built, even if the system is
./configure'd --with-system-readline and the build libreadline.a is not
used.

Proposed patch:

Fix ./configure.ac not to proceed readline/, when --with-system-
readline is provided

	* configure.ac: Don't configure readline if --with-system-readline is
	used.
	* configure: Re-generate.