builddeb: actually run make when building the deb (issue4778)

As of this change, we no longer produce broken debs, but I've already
got followups written that will produce much more standard-looking
packages and test the resulting packages.

contrib/builddeb

@@ -49,6 +49,7 b' sed "s/__VERSION__/$version/" < $control'
 mv $control.tmp $control
 
 if [ "$BUILD" ]; then
+    make PREFIX=$DEBBUILDDIR/usr install
     dpkg-deb --build $DEBBUILDDIR
     mv $DEBBUILDDIR.deb $DEBBUILDDIR/mercurial-$version-$release.deb
     if [ $? = 0 ]; then