bison: fix m4 dependency

Bison needs m4 (at runtime). In addition, the bison 2.5 configure script
checks for m4 4.1.6 or newer, and bails out otherwise, but ends up looking
at whatever the build host has rather than the cross version, which might
not be the correct version (or even available).

Fix it by short circuiting the check.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>