cdecl99: Fall back to getline, instead of Gnulib's readline.

cdecl99: Fall back to getline, instead of Gnulib's readline.

When building without readline, instead of using Gnulib's readline
replacement, use getline directly.  We can combine most of the batch
and interactive mode processing loops which saves a bunch of pointless
extra code in the readline-disabled case.

To actually do this, we prepare a Gnulib "local dir" to implement
a cut-down readline module that includes the configure tests only.