In the configure file, as this logic, it will take 2.5.4 smaller then 2.5.31

if perl -w <<EOF
    (\$min_version_major, \$min_version_minor, \$min_version_micro ) = "$flex_min_version" =~ /(\d+)\.(\d+)\.(\d+)/;
    (\$flex_version_major, \$flex_version_minor, \$flex_version_micro ) = "$flex_version" =~ /(\d+)\.(\d+)\.(\d+)/;
    exit (((\$flex_version_major > \$min_version_major) ||
     ((\$flex_version_major == \$min_version_major) &&
      (\$flex_version_minor > \$min_version_minor)) ||
     ((\$flex_version_major == \$min_version_major) &&
      (\$flex_version_minor == \$min_version_minor) &&
      (\$flex_version_micro >= \$min_version_micro)))
     ? 1 : 1);
EOF


Other information:
Cygwin