Use prebuilt m4 for bison too

Bison is currently looking for m4 directly in `/usr/bin/m4`, instead of
through $PATH, so it hadn't been tracked via the $PATH interposer
checks. But if you don't install m4 at all, it starts failing.

Bug: 117561006
Bug: 130111713
Test: Run on a machine without m4, build passes
Change-Id: I3fbad64127c183de79c17511e979fd5df946eb80