Compilation failure on config.cpp in Doxygen 1.8.6

Summary: update your GNU flex and try again.

When I tried to build the latest released Doxygen 1.8.6 today, I got a compilation failure on config.cpp.

I tried adding some global variable according to the error messages, and I worked it around successfully.

However, since it was generated from config.l and wasn't in git repository, I tried to update my flex from 2.5.4 to 2.5.33. Then automatic build works.

Later I found the Tools used to develop doxygen section of Doxygen Manual. It claims that GNU flex version 2.5.35 was used to develop and test Doxygen. Thus, updating flex makes sense.