system/qemu: Use MAKEFLAGS to pass numjobs.

We only hardcode -j1 if a build cannot work with multiple jobs,
otherwise rely on MAKEFLAGS.

https://idlemoor.gitlab.io/2016/09/15/Builds-that-need-j1.html

Signed-off-by: Andrew Clemons <andrew.clemons@gmail.com>

Signed-off-by: Willy Sudiarto Raharjo <willysr@slackbuilds.org>

system/qemu/qemu.SlackBuild

@@ -136,7 +136,7 @@ CXXFLAGS="$SLKCFLAGS" \
   $with_vnc \
   $targets
 
-make V=1 -j${NUMJOBS}
+make V=1
 make install DESTDIR=$PKG
 
 # Remove empty /var