qt: fix sysroot confusion in ./configure

qt's configure script is getting confused now that we're passing --sysroot=
in QMAKE_CC / QMAKE_CXX, causing misdetection of features and runtime
breakage.

Fix it by passing --sysroot in CFLAGS/CXXFLAGS instead.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>