Making non-standard OS checks in cmake?

[Raphael Kubo da Costa]

Hey there,

As you can see on my.cdash.org, both kdeadmin and kdenetwork fail to build 
cleanly on FreeBSD: 

 * kdeadmin/kuser's CMakeLists.txt's check for the need to link against -
lcrypt is broken on FreeBSD,  since the crypt() function is defined in 
unistd.h, not crypt.h, even though -lcrypt still must be passed to the 
compiler
 * kdenetwork/kppp has been broken since FreeBSD 8.0 - currently the ports 
system just disable its build, and I'd like to do the same upstream.

In both cases (the former can actually be solved in a different way, but I 
have felt the need for this in other cases as well), it would be useful to 
know if the current OS is FreeBSD, just like APPLE, WIN32 or UNIX are set. Is 
there an easy way to do that?