Add .pro files for qhull, qhulltest, rbox, and user_eg3.c

Qhull and rbox build correctly under mingw with gcc 4.4.0. Please
send email to bradb@shore.net if Qhull builds correctly with Linux
gcc 4.4.0 -O2. With gcc 4.2 and 4.3 qset.c segfaulted due to an
optimization error.

http://gcc.gnu.org/ml/gcc-bugs/2007-09/msg00474.html
[April 2008] of http://www.qhull.org/news/#bugs

user_eg3 is a demonstration of using Qhull C++ interface.

Qhulltest contains some failing testcases due to changes made for user_eg3.
Will be fixed soon.

qhull-qt reports an error because the link does not skip userprintf.c
in libqhull.a. Also the dependencies between qhull-qt and libqhull.a
are handled incorrectly.