Add SYSTEM_DOCTEST cmake option

Add SYSTEM_DOCTEST cmake option

This gives packagers option to use system version of doctest rather than
bundled one. It is disabled by default. Additionally, there is no need
to link rspamd-test-cxx with doctest as it is is single header library.

Main reason for this change is that currently bundled version of
doctest-2.4.5 can't compile with glibc-2.34 [1]. However, this issue was
already fixed in upstream doctest-2.4.6 [2] in commit [3]. With this,
packagers can just bump doctest to fixed version.

[1] https://github.com/onqtam/doctest/issues/473
[2] https://github.com/onqtam/doctest/blob/master/CHANGELOG.md#246-2021-03-22
[3] https://github.com/onqtam/doctest/commit/099d5414e97244ec44cf46b14cd176b3a3dc52e3