Release version 0.2.1

Added
* Linker's version script to limit exported symbols

Changed
* Configure now fails if linters are enabled and `cppcheck` is not
  available

Fixed
* Distribution archives
* Version in pkg-config file

Adjust of validation functions

Changed
* `base64_valid` renamed to `bas64_verify` and return value meaning
  changed
* `base64_str_valid` renamed to `base64_str_verify` and return value is
  now `size_t` (same as `base64_valid`).

First version of Base64C

* Initial version of base64c with unstable API for now