#8 calc upstream and licensing

[Don Porter]

Mpexpr is built on a (derivation of?) the
"calc" project.

https://sourceforge.net/projects/calc/
http://www.isthe.com/chongo/tech/comp/calc/index.html

Calc appears to have enjoyed a somewhat more
active history of bug fixes and releases. It's
natural when refreshing mpexpr to want to go get
a good set of calc updates.

A complicating factor is that at some point the
calc project was switched over to "version 2.1 of
the GNU Lesser General Public License". The impact
of that on our ability/desire to pull patches
from upstream is something I need other thoughts on.
Comments?

[Don Porter]

To make matters a bit worse, possibly, I haven't
been able to locate any distributions of calc
dating from before the license conversion.

The mpexpr README claims that
"David Bell's original 'calc' source can be found at:
ftp://ripem.msu.edu/pub/bignum/calc-2.9.3t6.tar.gz"
have expired.