xap_helper_cxx: support clang w/o `c++' executable
    
    This makes the C++ build work on Alpine Linux (tested 3.19.0)
    without having to install g++ to get the `c++' executable.
    I've tested this change with and without g++ on Alpine so it'll
    continue to work if a user decides to install g++.
    
    This should continue to work if the Xapian package on Alpine is
    changed to link against libc++ instead of libstdc++, since we
    only add `-lstdc++' as a fallback.  For reference, Xapian is
    already linked against libc++ and not libstdc++ on FreeBSD 13.x