[cairo] Cairo & pixman on PPC

[cu]

I finally got to bulding pixman 0.24 and cairo 1.10 on PowerPC (MacOS
10.4 SDK) - we've been using an old version until now.

A few issues popped up:

1. configure scripts for both pixman and cairo choke on linker warnings.
they seem to treat warnings like errors and fail any tests that rely on
those. It seems that *any* linker output is treated as an error. As a
result, testing for pthreads fails when linker comes up with an
unrelated warning. In particular, this makes pixman build without
"thread local" support (which otherwise is provided by
pthread_setspecific) and cairo not build at all if threading support is
requested. Pthreads are fully available on PowerPC MacOS.

2. Cairo seems to select "intel atomic primitives" on any platform where
it is built with GCC. I get this defined both on PPC and on ARM. It
definitely does not work on ARM/iOS and probably not on PowerPC Mac
either. Configure really should limit this define to true Intel systems.

3. There is a warning related to __visibility__ hidden attribute, as
used in cairo-compiler-private.h - this attribute does not work on
PowerPC Mac, evidently. The define depends only on version  GCC and
__APPLE__, probably should depend on apple and !powerpc.