[R] R and package don't know they're built on same system

[Ross Boylan]

I just had the interesting experience of building a package and R on
the same system, and having R refuse to load the resultant dynamic
library because it was thought to be for a different system.

The system was non-standard and beta, being a Linux-based 64 bit
Opteron system.  It uses the gnu tool chain.  The dynamic library was
built from C source.

When I tried to load the library R (1.8.1) complained
"package Rpmi was built for i686-pc-linux-gnu"
Inspection of R.version$platform, which is the thing the i686.... is
being compared to, shows that it is x86_64-unknown-linux-gnu.

I worked around this by removing the test causing the error (in
library.R), but, considering I built both R and Rmpi (an unofficial
version 0.4.6 from the author) within minutes of each other, it was
very surprising to find they had different notions of their system.

Any idea what's behind this, or how to fix it?