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

Bison older than 2.6 cannot compile the parsers. The definition '%define api.prefix' (introduced on 1fdc010392ffc) is not available on previous Bison versions (more references on the commit message). 

Error verified on:
 * Debian  7.1.0 (wheezy) shipped with Bison 2.5
 * Mac OS X 10.8.5 shipped with Bison 2.3 (!)

Running Bison 2.7.1 on the Mac there is no problem. On the other hand with Bison 3.0 there is another problem, I will open another issue for that.

Note that Debian Sid (unstable) and Ubuntu 13.10 (Saucy) are shipping Bison 2.7. 
I am not sure what is the situation on other distros, for this reason I think a version check is convenient.

Anyway, shall we move forward or keep compatibility with older systems?