[tlbuild] Inappropriate compiler flag for C++

[Andreas Scherer]

To whom it may concern:

When I './Build --disable-xdvik' on Linux with gcc/g++ 7.3.0, I get no
less than 101 warnings like

   cc1plus: warning: command line option '-Wimplicit' is valid
   for C/ObjC but not for C++

Where in the setup scripts is this compiler flag set, and more
importantly, would it be possible to remove/exclude it from the
'kpse_cv_warning_cxxflags' variable?

Thanks,
Andreas