Eliminate double download of bison v3.4.1 and v3.4.2

[Googulator]

These are very close versions, and most of their differences seem to come from how they are built (using custom makefiles, vs. using the normal build system). It seems unlikely that one or the other cannot be used in both steps where bison v3.4.x is required - and even if we need to keep both, we can probably save some download & srcfs bulk by only including 3.4.2's tarball, plus a patch to downgrade it to 3.4.1-equivalent.