The user may add a file: users_extra.c to have his own
additional functions added in a version independent way.

The distribution packages of Linrad contain a file extra.c
which is intended as a prototype. There is also a file
extra_w3sz.c which makes Linrad follow the frequency
selected in MAP-65.

After you have renamed extra.c or extra_w3sz.c to 
users_extra.c you have to run the configure script 
to update Makefile

Just copy your users_extra into any new Linrad version 
before running configure to include your own things in 
the new package