2.7.2 doesn't include any pentium specific optimizations
-m486 is still as good as it gets

heres a bit from the INSTALL file that might explain the bison troubles:
>  6. Make sure the Bison parser generator is installed.  (This is
     unnecessary if the Bison output files `c-parse.c' and `cexp.c' are
     more recent than `c-parse.y' and `cexp.y' and you do not plan to
     change the `.y' files.)

     Bison versions older than Sept 8, 1988 will produce incorrect
     output for `c-parse.c'.

-john