musescore: build fixes

Some translation files are installed or not installed depending on the
visibility of qt5-qttranslations in the build environment. For now,
simply explictly require this as a dependency. (It looks like there may
be more translation components to consider, but that's TBD separate
from basic build consistency.) Thanks to wiz@ for mentioning this.
Also, they've bumped the minimum GCC accepted from 4.7 to 4.8.

(gutteridge)