buildsystem: fix build for macOS

[tusharpm]

• use minimum required flex version 2.6 (Apple gives an older version built-in)
• fix linker flag for Apple

[nyx]

Nice. I had locally just hardcoded the APPLE conditional here with intent to conditionalize later. However if you'd prefer to keep parity there (at least my issue was with Clang not accepting those flags) consider using something like this which I believe is equivalent:

+       # for Clang the options are different
+       set_target_properties(nyan PROPERTIES LINK_FLAGS "-Wl,-undefined,error")

[tusharpm]

Thanks for the review. 👍

About the specification of the link flag: I have that in my first commit. However, in my opinion, these flags are better left as implicit defaults if they're favorable. On macOS, man ld states "[...] The default is to treat undefined symbols as errors."
I usually avoid specifying platform/tool-specific flags if possible. If the explicit specification solves some issue, I can remove my second commit (assuming the first one is acceptable, of course).

[nyx]

@tusharpm OK - that's fine by me. It seems doubtful that the defaults there will change any time soon, and I favor minimalism. Maybe note that in a comment?

[TheJJ]

should do.