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

Commit 228cf97dd3c8 ("Merge configure.ac from gcc project") undid the
change originally done in commit 69961a84c9b ("Don't build
readline/libreadline.a, when --with-system-readline is supplied").
Re-apply it.

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