Adjust Makefile to be consistent with qcsrc/Makefile
Makefile and qcsrc/Makefile behave differently:
in xonotic-data.pk3dir, make
make all
and make qc
all do the same thing: build the qcsrc and bundle the csprogs.dat into a pk3.
in xonotic-data.pk3dir/qcsrc, make
and make all
build the qcsrc and the csprogs pk3, and make qc
does the build but skips creating the pk3.
This change makes the behaviour consistent, and allows release builds to skip creating the pk3 by using make qc
.