Check out this thread on their discourse server:

https://discourse.trueos.org/t/solved-fresh-install-no-c-compiler/3548

> Thanks, harv .   I installed Trident now.  I'm not able to do much with  
> pkg search
> 
> like I usually can when using TrueOS or FreeBSD.   I got vim, emacs and gcc
> installed, but not clang.   It doesn't find clang in the repositories when
> I type: pkg install clang.
> When I try to build with gcc, I get:
> fatal error: string.h: No such file or directory.
> 
> Maybe there's a development package I need to install, but I'm not
> sure what the name of it is.  There are some things I like about Trident,
> so I hope I can figure out how to solve these problems.   Thanks for
> letting me know about this.
> 
> 
> Brian
> Ebenezer Enterprises - In G-d we trust.
> https://github.com/Ebenezer-group/onwards