Making flex a bootstrap tools to avoid build breakage going from 9.* to 10

[roberto]

Summary

10 has a new flex (2.5.37) and the config.h for unbound has been updated to
take this into account. Alas it breaks source upgrade from any version of
9 because flex is not built as a bootstrap-tools (it would be for older
versions).

That means "libunbound/configlexer.c" is built with the old flex but using
config.h for the new one. Build is thus broken going from 9.* to 10.

Make flex a bootstrap-tools entry if host is less than 1000033 to take into
account flex update in 10.

Test Plan

Tested on both 9.2-RC3 and 9.3 by myself and dim@. Running buildworld in
head but as both 10 and 11 has the new flex, it will not matter.

[imp]

Looks like the right fix to me for the problem you described.

For bonus points, please sort this so the bootstrapping if statements are in strictly increasing order. This will make my life easier in the future. Thanks!

[roberto]

imp: will sort as you requested. The window of breakage is limited to the time between the new flex import and now but as source upgrades are still supported, we need to fix this. Will merge to stable/10 ASAP.

[roberto]

Sorting entry as asked by imp.

[roberto]

Committed as r269662.