qtbase: disable fontconfig support by default

Currently fontconfig support is autodetected. It is better to be
deterministic. Since qtbase-fonts is not installed where fontconfig
will find it, it seems that current users are not using fontconfig.

Signed-off-by: Philip Craig <phil@blackmoth.com.au>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>