_ptr and _srcptr types

[Torbjörn Granlund]

Let's not forget that portability in the real world is far more
important than conformance to this or that standards document.
Standards are useful to achieve the former goal only.

We're pretty good at it: https://gmplib.org/devel/tm/gmp/date.html

GMP will never be ported to run on either any C standard document nor
any C++ standard document.  But we won't argue against Vincent casting
the terminating mpz_inits argument to mpz_ptr (but hopefully he refrains
from doing that until we solemnly declare that we have pronounced it as
part of the GMP standard).  :-)

-- 
Torbjörn
Please encrypt, key id 0xC8601622