There was no good documentaion but in fact was realy easy.
1) get sources rfunc-2.1.3.1-RC1-unix.tar.gz
2) untar it:  

#tar -xzf rfunc-2.1.3.1-RC1-unix.tar.g 
#cd sources

3)change config file ‘makefile.freebsd’: here is my config makefile
4) run make :

#make -f makefile.freebsd rfunc

5)copy ‘rfunc’ to ‘/usr/local/libexec/firebird/udf/rfunc.so’

#cp rfunc /usr/local/libexec/firebird/udf/rfunc.so

(found some useful information on this forum post )