Fix 'make install' on OSes where 'python3' doesn't exist.

Sometimes the Python 3 executable is called just 'python3', sometimes
it is called 'python3.4' and so on. Handle all cases transparently for
the user.

Before:

  $ make install
  /usr/bin/env: python3: No such file or directory

After:

  $ make install
  python3.4 ./tools/install-decoders -i ./decoders -o [...]
  Installing 36 protocol decoders:
  [...]