Add SecBSD support

[michaelortmann]

Found by: michaelortmann
Patch by: michaelortmann
Fixes:

One-line summary:
Add SecBSD to aclocal.m4 so it drops WEIRD_OS mark and builds dynamic by default

Additional description (if needed):
Please run misc/runautotools when merging

Test cases demonstrating functionality (if applicable):

$ ./configure
[...]
Operating System: SecBSD 1.6
IPv6 Support: yes
Tcl version: 8.6.16 (threaded)
SSL/TLS Support: yes (LibreSSL 4.0.0)
Threaded DNS core: yes

If you experience any problems compiling Eggdrop, please read the
compile guide, found in doc/COMPILE-GUIDE.

Type 'make config' to configure the modules, or type 'make iconfig'
to interactively choose which modules to compile.

$ make install
[...]
Installation completed.