Fix readline detection with a non-standard LOCALBASE

Due to a bug in m4/lftp_lib_readline.m4, a wrong prefix is found for readline
when using a non-standard LOCALBASE [1]. Add a temporary workaround waiting for
the patch to be merged upstream [2].

https://reviews.freebsd.org/D15731 [1]
https://github.com/lavv17/lftp/pull/458 [2]

Differential Revision: https://reviews.freebsd.org/D15731