Fix cross-compilation for `vpnc'.

The error was because the `Makefile' tried to execute the resulting binary and
failed as a result. This is done to generate the manpage.

The solution I found was to compile twice the binary, one for the host and the
other for the target, this way the manpage is generated and installed first and
the binary for the target is built later. When not cross-compiling this is not
done.

The error can be seen by executing:

guix build vpnc \
           --target=aarch64-linux-gnu

I'm not entirely sure that I've made a correct git message, so feel free to
modify it if doesn't match the standards.


—
Jean-Pierre De Jesus DIAZ