Remove flex version checks

Remove the flex version checks from configure and meson.  The cutoff
versions are all so ancient that this is no longer relevant, and what
the actual cutoff should be is a bit fuzzy.

This also removes the ancient behavior that configure would also
accept a "lex" program if it is actuall flex.  This aligns the check
with meson in this respect.

For future reference, as of this commit, these are relevant flex
versions:

- The hard required minimum is flex 2.5.34 as of commit b1ef48980d,
  but this has not actually been tested.

- Prior to this, the minimum enforced by configure/meson was flex
  2.5.35, which is the oldest present in the buildfarm right now.

- As of commit 6fdd5d9563, the oldest version that will compile
  without warnings due to flex-generated code is flex 2.5.36.

- The oldest version that probably still has some practical relevance
  is flex 2.5.37, which ships with CentOS/RHEL 7.

Discussion: https://www.postgresql.org/message-id/1a204ccd-7ae6-478c-a431-407b5c48ccc6@eisentraut.org

configure

@@ -9931,59 +9931,68 @@ if test -z "$BISON"; then
 fi
 
 
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for flex" >&5
-$as_echo_n "checking for flex... " >&6; }
-if ${pgac_cv_path_flex+:} false; then :
+if test -z "$FLEX"; then
+  for ac_prog in flex
+do
+  # Extract the first word of "$ac_prog", so it can be a program name with args.
+set dummy $ac_prog; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_FLEX+:} false; then :
   $as_echo_n "(cached) " >&6
 else
-  # Let the user override the test
-if test -n "$FLEX"; then
-  pgac_cv_path_flex=$FLEX
-else
-  pgac_save_IFS=$IFS
-  IFS=$PATH_SEPARATOR
-  for pgac_dir in $PATH; do
-    IFS=$pgac_save_IFS
-    if test -z "$pgac_dir" || test x"$pgac_dir" = x"."; then
-      pgac_dir=`pwd`
-    fi
-    for pgac_prog in flex lex; do
-      pgac_candidate="$pgac_dir/$pgac_prog"
-      if test -f "$pgac_candidate" \
-        && $pgac_candidate --version </dev/null >/dev/null 2>&1
-      then
-        echo '%%'  > conftest.l
-        if $pgac_candidate -t conftest.l 2>/dev/null | grep FLEX_SCANNER >/dev/null 2>&1; then
-          pgac_flex_version=`$pgac_candidate --version 2>/dev/null`
-          if echo "$pgac_flex_version" | sed 's/[.a-z]/ /g' | $AWK '{ if ($1 == 2 && ($2 > 5 || ($2 == 5 && $3 >= 35))) exit 0; else exit 1;}'
-          then
-            pgac_cv_path_flex=$pgac_candidate
-            break 2
-          else
-            as_fn_error $? "
-*** The installed version of Flex, $pgac_candidate, is too old to use with PostgreSQL.
-*** Flex version 2.5.35 or later is required, but this is $pgac_flex_version." "$LINENO" 5
-          fi
-        fi
-      fi
-    done
+  case $FLEX in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_FLEX="$FLEX" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_FLEX="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
   done
-  rm -f conftest.l lex.yy.c
-  : ${pgac_cv_path_flex=no}
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+FLEX=$ac_cv_path_FLEX
+if test -n "$FLEX"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $FLEX" >&5
+$as_echo "$FLEX" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
 fi
 
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_path_flex" >&5
-$as_echo "$pgac_cv_path_flex" >&6; }
-if test x"$pgac_cv_path_flex" = x"no"; then
-  as_fn_error $? "flex not found" "$LINENO" 5
+
+  test -n "$FLEX" && break
+done
+
 else
-  FLEX=$pgac_cv_path_flex
-  pgac_flex_version=`$FLEX --version 2>/dev/null`
-  { $as_echo "$as_me:${as_lineno-$LINENO}: using $pgac_flex_version" >&5
-$as_echo "$as_me: using $pgac_flex_version" >&6;}
+  # Report the value of FLEX in configure's output in all cases.
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for FLEX" >&5
+$as_echo_n "checking for FLEX... " >&6; }
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $FLEX" >&5
+$as_echo "$FLEX" >&6; }
 fi
 
+if test -z "$FLEX"; then
+  as_fn_error $? "flex not found" "$LINENO" 5
+fi
+
+pgac_flex_version=`$FLEX --version 2>/dev/null`
+{ $as_echo "$as_me:${as_lineno-$LINENO}: using $pgac_flex_version" >&5
+$as_echo "$as_me: using $pgac_flex_version" >&6;}
+