Have configure.ac tolerate unregistered builds or new platforms

[Gabriel Dos Reis]

Hi,

GHC's configure.ac is at the moment pretty intolerant with
respect to unregistered builds: it will issue outright rejection
for triplets it does not know about.  However, the first thing
we want to do when performing an unregistered build or porting to
a new platform (or even cross-compiling) is to run configure
and inspect what it founds.  For that to happen, configure.ac
ought to be flexible.  Here is a patch that implements just that:

   https://github.com/sigfpe/ghc/commit/1c0e32be28961afc63b9eae1add22bd14e11a3cd

What it does:
  -- removal of checkVendor
     this check isn't doing anything really useful.
     GHC should not halt configuration just because
     it does not know about an OS vendor.

  -- 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.

-- similar changes need to be reflected in GHC_CONVERTT_CPU,
    GHC_CONVERT_VENDOR, GHC_CONVERT_OS.

Thanks,

-- Gaby