Adding a build-dependency on flex

[David Nolden]

I'm also against adding dependencies just to save very few developers
from an infrequent additional command.

There are more dangers to dependencies than just the inconvenience,
for example different versions of the dependency might be installed on
some machines, and might generate incompatible output, which a
developer not familiar with the code in question can hardly deal with
(I've been there before with bison).