2.x require GCC 4.8

MuseScore 2.1 (and later 2.x versions) need GCC 4.8 because they use a C++11 feature not correctly handled by GCC 4.7 yet:


In file included from /<>/musescore-2.1.0+dfsg3/libmscore/pos.h:23:0,
from /<>/musescore-2.1.0+dfsg3/awl/poslabel.h:24,
from /<>/musescore-2.1.0+dfsg3/awl/poslabel.cpp:21:
/<>/musescore-2.1.0+dfsg3/libmscore/sig.h:45:23: error: ‘Ms::Fraction::Fraction’ names constructor
make[4]: *** [awl/CMakeFiles/awl.dir/poslabel.cpp.o] Error 1


https://github.com/musescore/MuseScore/pull/3589 is the trivial fix; targetting 2.2.1 but merge at your own deliberation.