Allow to use system's aglfn.txt and regenerate /src/gui/text/qfontsubset_agl.cpp at build time

I have received a bug report [0] to use the aglfn.txt as shipped by Debian and regenerate /src/gui/text/qfontsubset_agl.cpp at build time. While I can do this in the packaging itself, it may be worth to try to add a method for doing this directly in upstream, which could be used by others.

If you are interested, I can try to make a patch to achieve this. Else it would be good to know that you are not interested in this.

[0] <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=709586>

Kinds regards, Lisandro.