accessibility/qt5-speech: add options SPEECHD and FLITE

Previously we only assumed to build the speechd backend. If however flite was
installed, it would be picked up and could leed to it failing due to a missing
dependency to qt5-multimedia required by said backend.

With this we add options to deterministically toggle which of the backends to
build.

PR: 235900
Reported by: starikarp@dismail.de, lumiwa@gmail.com