package/mpd: add choice between version 0.19 and 0.20

MPD version 0.20 being affected by GCC PR libstdc++/64735 means no mpd package
available in Buildroot for the architectures NIOSII, ARMv4, ARMv5 and SPARCv8
until GCC 7 is released.

As the next Buildroot release is in 2017.02, which is before GCC 7 is expected
to be finished we add hidden config symbols to select the version branch to use.

Note, that we bumped the version from 0.19 to 0.20 quite recently within the
current Buildroot release cycle. This way we can keep the support for mpd on
the affected architectures, like ARMv5, until GCC 7 is out.

Note, that for the version 0.19 MPD requires libglib2.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>