bug#44795: automake-1.16.3 make check failed on Solaris 11.3 x86/x64

[Karl Berry]

    The same errors are detected with gcc-9.3.0.

Ok, gcc 9 or 10. We can figure out the exact version where gcc started
the errors if we need to. It doesn't happen with the much older gcc
version I have (reports as gcc 4.8.5 on centos7, though centos does
major patches to that).

Anyway, I guess the issue is that this two-line C++ program no longer
compiles with new-enough gcc:

  extern "C" int yylex (void);
  extern int yylex (void);

Can you discern what change to the code will pacify the compiler?
I could not find the answer.

(The "C" line is in the test sources. The non-"C" line is generated by
(f)lex.)

Thanks,
Karl