[OE-core] question about meta/class/qmake-base.bbclass

[Kang Kai]

Hi Holger,

When I read the file  meta/class/qmake-base.bbclass, I find that that 
are 2 lines:
     unset QMAKESPEC || true
     ${OE_QMAKE_QMAKE} -makefile -spec ${QMAKESPEC} -o Makefile 
$QMAKE_VARSUBST_PRE $AFTER $PROFILES $QMAKE_VARSUBST_POST || die "Error 
calling ${OE_QMAKE_QMAKE} on $PROFILES"

I am confused that why unset QMAKESPEC first and then use it a argument 
for qmake?
Could you help me to get out of that?

Thanks and Regards,
Kai