Add ability to list files for which warning 4 is disabled

Since we'd like to enable the 'fragile pattern matches' warning, but
it's hard to get rid of it in some files (mostly pattern-matching on
`Unix.error`), we need a way to disable to corresponding warning for
these specific files.

This change adds the feature to `myocamlbuild.ml`, not whitelisting any
file for now.
Merge pull request #458 from Incubaid/allow-fragile-pattern-matches

Add ability to list files for which warning 4 is disabled