pinentry-qt4: make the accessibility part optional

Check if the Qt libraries have support for QT Accessibility before using
it. Otherwise it will raise error like these one:

main.cpp: In function 'int qt_cmd_handler(pinentry_t)':
main.cpp:220:51: error: 'class QAbstractButton' has no member named
'setAccessibleDescription'

• Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>