compiler flags files, since they essentially duplicate the `error-`
files modulo the replacement of `-Werror=` with `-W` and any changes in
comments.  (I verified the duplication with a script.)

For autoconf, reinstate the use of the `demote_errors` shell function to
derive the `noerror-` content from the `error-` content.  `demote_errors`
replaces `-Werror=` with `-W` when `WARNINGS_AS_ERRORS` is `no`.

Slightly reorder `configure.ac` so that the setting of
`WARNINGS_AS_ERRORS` takes effect before the `error-` files are sourced.

the autoconf files to remove `noerror-` files.  I'm not much of a CMake
user so these changes are quite rough.

Looks like the duplication can be reduced with the introduction of a new
macro.

HDF5_ENABLE_WARNINGS_AS_ERRORS test once in the ADD_H5_FLAGS macro.