RFC: don't use strncmp

[Akim Demaille]

This commit changes the behavior of Bison on when used
with -o foo_tab.c.  Before, it would create foo.output,
recognizing that _tab is the same as .tab, and therefore
should be omitted, now it would create foo_tab.output.
In other words, I don't think that, today, our Unix
platforms should care about foo.tab.c not being valid on
M$ DOS.

But I'll wait for a go before installing.