scripts/package: don't break if %{_smp_mflags} isn't set

Currently, if we do a 'make rpm-pkg' without the _smp_mflags rpm macro
defined, the build fails with:

[snip]
Executing(%build): /bin/bash -e /var/tmp/rpm-tmp.67959
+ umask 022
+ cd /home/jk/devel/kernel-snapshot/rpm/BUILD
+ cd kernel-2.6.26
+ make clean
+ make '%{_smp_mflags}'
make[3]: *** No rule to make target `%{_smp_mflags}'. Stop.
error: Bad exit status from /var/tmp/rpm-tmp.67959 (%build)

This change uses the 'null if not set' reference to the _smp_mflags
macro instead.

Signed-off-by: Jeremy Kerr <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>

scripts/package/mkspec

···
 echo "%build"
 
 if ! $PREBUILT; then
-echo "make clean && make %{_smp_mflags}"
+echo "make clean && make %{?_smp_mflags}"
 echo ""
 fi