[tlbuild] Inappropriate compiler flag for C++

[Karl Berry]

Hi Andreas,

    When I './Build --disable-xdvik' on Linux with gcc/g++ 7.3.0
    ...
    cc1plus: warning: command line option '-Wimplicit' ...

Hmm. From what I can see in m4/kpse-warnings.m4, -Wimplicit should only
be included in kpse_cv_warning_cxxflags if the output of g++
-dumpversion matches globs 3.* or 4.[012345].* which is presumably not
the case for 7.3.0.  I don't understand.

Can you run sh -vx .../Build/source/configure and look for wimplicit in
the (voluminous) output and see if you can discern why it is being set?

Or maybe for some reason the kpse_cv_warning_cflags is being applied,
rather than cxxflags, though this should not be the case.

Personally, I just compile with --enable-compiler-warnings=no.

Best,
Karl