Fix compilation with >=flex-2.6.1
flex devs reverted yyleng's type back to int in 2.6.1 [1].
It was previously changed from int to yy_size_t in 2.5.36 [2,3].
Adjust the related guard accordingly.
See also https://bugs.gentoo.org/show_bug.cgi?id=579490
1: https://github.com/westes/flex/commit/7a7c3dfe1bcb8230447ba1656f926b4b4cdfc457
2: https://github.com/westes/flex/commit/9ba3187a537d6a58d345f2874d06087fd4050399
3: ada178bd