Let Singular build with clang.
Trac #12442: Singular does not state return type of main() functions

This breaks building with clang.

I've posted an spkg fixes this (and #12441) at
[http://wstein.org/home/ohanar/clang-
port/sage-5.0.beta1-src/spkg/standard/singular-3-1-3-3.p5.spkg]. (This
is a review spkg in case there are more issues with clang down the
road.)

URL: http://trac.sagemath.org/12442
Reported by: ohanar
Ticket author(s): R. Andrew Ohana, Leif Leonhardy
Reviewer(s): Leif Leonhardy, Jean-Pierre Flori, Volker Braun