It's not an LMMS bug because my festival headers are the same as yours and there's no problem (I don't have the Makefile though). Anyway, do check that you have the following:

For the translation thing prepend the env var:

QT_TRANSLATIONS=/opt/qt ./configure --with-qtdir=/opt/qt

Provided your qt is not in /usr. So for the main problem you need to consult your distribution and make sure you really have the development files.