This allows users to set e.g. `extra_compile_args=["-g"]` (temporarily,
for debugging purposes) and not have that get overridden by
Pybind11Extension's default `-g0`.

In order to minimize "order inversion" (not that it really matters,
though) I chose to collect all flags and add them at once (hence the
dropping of `*args`).  I also dropped flag deduplication, which seems
unneeded.