Add version check for Bison, mininum 2.6

Included m4 macro the test Bison min version. Set min version to 2.6.

The definition 'api.prefix' lead to an error with an older Bison.

 * preprocessorYacc.y:1.9-18: syntax error, unexpected identifier, expecting string

Acording to the manual [1] this feature is only present after 2.6.

[1] http://www.gnu.org/software/bison/manual/html_node/Multiple-Parsers.html
Update autogenerated configure scripts.

Run aclocal; autoconf; automake --add-missing;
Merge pull request #5 from guitorri/check-bison

Check Bison version, min 2.6 to handle api.prefix definitions.