Don't build readline's shared libs by default

Don't build readline's shared libs by default

Since the sync to version 7.0-alpho, readline now by default builds
(unused) shared libraries alongside static libraries, whereas before it
only built static libraries.  A couple of GDB buildbots were not happy
with this change:

  http://gdb-build.sergiodj.net/builders/AIX-POWER7-plain/builds/240
  http://gdb-build.sergiodj.net/builders/Fedora-i686/builds/1518

To get these buildbots building again, this patch alters readline's
configure.ac file to not build shared libraries by default, as was the
case with readline 6.2.  A more permanent fix may be to alter the
top-level Makefile.def to pass --disable-shared to readline, or to
investigate why these building these shared libraries are giving the
buildbots trouble.  (I think the proximate reason why the i686 buildbot
fails is because it passes CFLAGS=-m32 instead of CC="gcc -m32" to the
top-level configure script, and readline's linker commands don't inherit
CFLAGS.  Not sure about the AIX failure.)

readline/ChangeLog.gdb:

	* configure.ac: Default opt_shared_libs to no.
	* configure: Regenerate.