User flags should be passed via CFLAGS/CXXFLAGS/LDFLAGS,
or --extra-cflags/extra-cxxflags/--extra-ldflags on the
command line.

QEMU_CFLAGS, QEMU_CXXFLAGS and QEMU_LDFLAGS are reserved
for flags detected by configure, so do not add to them
and clear them at the beginning of the script.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>