[tlbuild] Inappropriate compiler flag for C++

[Karl Berry]

    where AM_CXXFLAGS is prepended by $(WARNING_CFLAGS), thus adding the
    non-C++ option '-Wimplicit'. '-Wno-mismatched-tags' comes from the
    original sources (version 2.6.2):

Thanks (and for reporting to Martin). I adjusted our src/Makefile.am and
patch. -k