Fix checked builds with gcc4.2

Fix checked builds with gcc4.2

Recent change sets removed the 'ignore-all-warnings' mode from a
number of files, in favour of using the implicit-function-declaration
pragma. Sadly, that pragma isn't available in gcc 4.2. Revert the
Makefile changes such that ignore all warnings is turned back on
for the affected files, and note this in README.WARNINGS

Reviewed-on: http://gerrit.openafs.org/639
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>