re: Discussion Unidata#2554
re: PR Unidata#2231
re: Issue Unidata#2189

After some discussion, the issue of applying filters on variables
whose type is not fixed size, was resolved as follows:
1. A call to nc_def_var_filter will ignore such filters, but will issue a log warning.
2. Loading (from an existing file) a variable whose type is not fixed-size and which has filters, will cause the variable to be suppressed.

This PR enforces those rules.

### Misc. Other changes
* Add a test case to test the vlen change.
* Make some minor clean-ups in various cmake and automake files.
* Remove unused test