configure: check that the version of emacs we may have is at least 24

This also effectively drops support to Emacs version 23, which was
deprecated in version 0.23.2 (2016-11-20) and did not entirely work
any more.

diff --git a/configure b/configure
index b2200be08a03069957c76af3a59f3fbbffd360d6..3ae26241df2a9d16fa6b319ad963bdf2af56fb92 100755 (executable)
--- a/configure
+++ b/configure
@@ -623,8 +623,8 @@ if [ -z "${EMACSETCDIR-}" ]; then
     EMACSETCDIR="\$(prefix)/share/emacs/site-lisp"
 fi
 
-printf "Checking if emacs is available... "
-if emacs --quick --batch > /dev/null 2>&1; then
+printf "Checking if emacs (>= 24) is available... "
+if emacs --quick --batch --eval '(if (< emacs-major-version 24) (kill-emacs 1))' > /dev/null 2>&1; then
     printf "Yes.\n"
     have_emacs=1
 else