Accept flex > 2.5.x in configure.

Per buildfarm member anchovy, 2.6.0 exists in the wild now.
Hopefully it works with Postgres; if not, we'll have to do something
about that, but in any case claiming it's "too old" is pretty silly.

config/programs.m4

@@ -69,7 +69,7 @@ else
         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 && [$]2 = 5 && [$]3 >= 31) exit 0; else exit 1;}'
+          if echo "$pgac_flex_version" | sed ['s/[.a-z]/ /g'] | $AWK '{ if ([$]1 = 2 && ([$]2 > 5 || ([$]2 = 5 && [$]3 >= 31))) exit 0; else exit 1;}'
           then
             pgac_cv_path_flex=$pgac_candidate
             break 2

configure

@@ -7164,7 +7164,7 @@ else
         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 && $3 >= 31) exit 0; else exit 1;}'
+          if echo "$pgac_flex_version" | sed 's/[.a-z]/ /g' | $AWK '{ if ($1 = 2 && ($2 > 5 || ($2 = 5 && $3 >= 31))) exit 0; else exit 1;}'
           then
             pgac_cv_path_flex=$pgac_candidate
             break 2