Check 'mpg123' is installed.

Audio::Play::MPG123 is only frontend of mpg123, so it does not assure
that 'mpg123' is already installed.

If people does not install `mpg123`, they got following message by `perl Build.PL`.
````
NA: Unable to build distribution on this platform.
Locating required external dependency bin: mpg123... missing.
Unresolvable missing external dependency.
Please install 'mpg123' seperately and try again.
````