When creating a new `Makefile` target to build,
it's also necessary to update the `clean` target,
which purpose is to remove built targets when they are present.
This process is simple, but it's also easy to forget :
since there is a large distance between the position in the `Makefile` where the new built target is added,
and the place where the list of files to `clean` is defined.
Moreover, the list of files becomes pretty long over time,
hence it's difficult to visually ensure that all built targets are present there,
or that no old target (no longer produced) is no longer in the list
This PR tries to improve this process by adding a CLEAN variable.
Now, when a new built target is added to the `Makefile`,
it should preceded by :
```
CLEAN += newTarget
newTarget:
<TAB> ...recipe...
```
This new requirement is somewhat similar to `.PHONY: newTarget` for non-built targets.
This new method offers the advantage of locality :
there is no separate place in the file to maintain a list of files to clean.
This makes maintenance of `make clean` easier.
* Removing symbols that are not being tested
* Removing symbols used in zstdcli, fileio, dibio and benchzstd
* Removing symbols used in zbuff and add test-zbuff to travis
* Removing remaining symbols and adding unit tests instead
* Removing symbols test entirely
