qscintilla2: Use $(SOURCEFORGE_MIRROR)

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
qt*: Disable i686-pc-mingw32

Leftover from 8b70bf0008c8c70957adde14082dee25f78b1b51

Signed-off-by: Timothy Gu <timothygu99@gmail.com>