build: Use gnulib module to get readline libraries. * bootstrap.conf: Import readline module. * configure.ac: Remove separate checks for terminal library. Use readline linker flags as set by gnulib module. * m4/acinclude.m4 (OCTAVE_CHECK_LIB_TERMLIB): Remove function that is no longer needed. (OCTAVE_ENABLE_READLINE): Replace custom check by calling gnulib function. No longer check if version is at least 4.2. (It was released 20+ years ago.) * build-aux/subst-config-vals.in.h, build-aux/subst-cross-config-vals.in.h: Substitute READLINE_LIBS with result from gnulib function. Remove TERM_LIBS. * libinterp/build-env.h, libinterp/build-env.in.cc, libinterp/corefcn/toplev.cc: Remove TERM_LIBS. See also: https://octave.discourse.group/t/3559

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