wrap git dependencies in cmake toggles

Using current master, c895049 , since 6.3.9 there are a number of dependencies
that try to fetch code at build time. As FreeBSD ports system blocks network access
during build phase, I'd like to wrap these in something like:

```
if(NOT WITHOUT_FLOWBENCH)
...
endif()
```

Which should allow whatever existing CI / docker stuff to run without change, and allow downstream packagers to work around this. Would these be acceptable?

- [x] flowbench - simply patched it out at present
- [x] doctest - patched out as I haven't found a recipe to get FDB to find and use existing doctest port
- [x] toml11 - resolved by adding toml11 as a port

If so, I'll do a PR with doctest & flowbench toggles.

Locate the CMake dependency setup for flowbench, doctest, and toml11, then inspect how the existing CI and Docker builds invoke it. Done means FreeBSD-style network-restricted builds can disable the relevant fetches through toggles while the default CI and Docker behavior remains unchanged.

Written by the indexing model from the issue text.