Have configure.ac tolerate unregistered builds or new platforms

[Ian Lynagh]

On Mon, Jan 16, 2012 at 12:42:19PM -0600, Gabriel Dos Reis wrote:
> 
>   -- have both checkArch and checkOS not abort configuration
>      they should normalize arch and os known to be troublesome
>      and report ArchUnknown and OsUknown for arches and OSes
>      not known to GHC.

The rationale for the current behaviour is that it will catch typos and
incorrect arch names (e.g. using x86 rather than i386). The assumption
is that people will make typos more frequently than there will be ports
to new arches (as there are a limited number of arches to which one
could port), and that it's more useful to catch the typos (I'm not sure
OTTOMH what happens if you typo otherwise; you might end up with a
complete unregisteriesd build. By contrast, when porting to a new
platform, configure stops immediately and tells you what's wrong).


Thanks
Ian