rely on qmake's CXXFLAGS; rel 2

rely on qmake's CXXFLAGS; rel 2

passing CXXFLAGS in environment overrides flags added by qt completely
resulting in build failure due to missing -fPIC. while it's possible to
add additional flags by means of:

qmake 'QMAKE_CFLAGS+=${QMAKE_CFLAGS}'

they're always prepended, never appended so qmake's flags take
precedence anyway. since build time %{rpmcxxflags} are already part of
qmake's flags let's just rely on them.