configure: Add base check for dynmodules parameter to prevent impossible build attemp...

configure: Add base check for dynmodules parameter to prevent impossible build attempts due to invalid values

We didn't validate dynmodules parameter values which allows build failures like

> [...]
> Making all in geobackend
> /bin/sh: line 20: cd: geobackend: No such file or directory
> Makefile:499: recipe for target 'all-recursive' failed
> make[2]: *** [all-recursive] Error 1

which happens for example if a dynamic module was removed (or the user just passed an
invalid value).

Now we will do a base check by checking for the existence of the module's source dir.
This will allow us to catch errors like that and show a meaningful error message if
necessary.

Reference: https://github.com/PowerDNS/pdns/issues/3171