Note that this patch introduces some significant changes to the recipe,
and I'm unsure about their correctness.  (It builds fine, but maybe
isn't future-proof.)

AFAIUI, python, flex, bison, and gettext are all only needed (at build
time) when rebuilding parts of the build system.  Maybe they were
left-overs from before commit ce4d7ab which adds a hack to prevent the
build system from rebuilding parts of itself; however the same commit
*adds* gettext which wasn't there before.  Maybe it was an oversight,
and the commit shouldn't had added gettext and should have removed the
other three from the native inputs?

In any case, it builds fine when I remove said hack (doesn't seem
necessary anymore), and also remove all four from native inputs.

(Actually there are also some .py files in the source tree which I'm
told might be necessary at build time regardless of rebuilding the build
system, but this doesn't seem to be the case for us, maybe because
they're only needed for some optional features which we don't have
enabled.)

Input from any mesa experts or those who touched this package before is
welcome.