elisp: my-lang: make my-compile aware of prefix arg

When type C-u F9 or C-u C-c C-c, use the prefix to ask the user for the
command to use, just as (compile) does.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
init: source host specific configurations

Source each file specific to a host depending on its hostname. For
example, for a host named belgarion :
 - (load ".emacs.d/elisp/my-host-belgarion*.el")

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>