Add support for warning checked builds

Add support for warning checked builds

This patch adds a '--enable-checking' configuration option. When this
option is supplied, and gcc is in use, the compiler will treat any
warnings as errors. This will hopefully help stop new warnings from
creeping into the tree.

In order to still be able to build, all of the currently existing
warnings are accepted (these are documented in README.WARNINGS). With
this set of warning inhibitions, the tree is known to build on 32bit
Leopard - other systems may vary. Warning inhibition may be disabled
by supplying --enable-checking=all - in this case the tree will
definitely not build!

If --enabled-checking is not specified, the existing compilation
behaviour is maintained, so there is no user-visible change.

Gcc 4.2, or later, is required to use the pragma sets contained within
this patch. Again, they are not visible unless --enable-checking is
given.

Reviewed-on: http://gerrit.openafs.org/526
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>