build: require system readline (or Tcl); drop the bundled fallback (Patch 1)

configure already preferred the system readline (AC_CHECK_LIB → -lreadline) and
only fell back to the bundled readline/ copy.  Remove that fallback: a non-Tcl
interactive build now requires the *system* GNU readline -- both its header
(AC_CHECK_HEADER readline/readline.h, so configure fails cleanly instead of deep
in compilation) and the library -- and if neither Tcl nor readline is available
configure errors with a clear explanation of the three ways forward (build with
Tcl, install libreadline-dev, or --disable-readline).

Also drop the now-meaningless --enable-readline-bundled, and substitute
MAKE_READLINE=0 unconditionally so the Makefiles' bundled-readline path goes
inert.  readline/ and all Makefiles are left untouched here (cleaned up and
removed in the following patches).

--disable-readline stays as the escape hatch: it just tells configure to ignore
the system readline and build the existing no-line-editing variant.

Verified: Tcl build rc=0 (readline off, 375 files, readline/ not built); non-Tcl
--disable-readline builds (serial); non-Tcl default with no readline dev package
now errors with the guidance message.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

.github/workflows/canary-matrix.yml

@@ -153,6 +153,11 @@ jobs:
 
           #
           _configure_args=()
+          # "none" installs no dev packages (no libreadline-dev among them).  Since
+          # configure now requires system readline for a non-Tcl build (unless told
+          # otherwise), the minimal "none" build must pass --disable-readline.
+          if [ "${MATRIX_PKGS}" = "none" ]; then
+            _configure_args+=(--disable-readline); fi
           if echo -n "$MATRIX_PKGS" | grep -q "^no.*_zl"; then
             _configure_args+=(--disable-compression); fi
           if echo -n "$MATRIX_PKGS" | grep -q "^no.*_brl"; then