xgettext invocation and docs

[Andrea G. Monaco]

Hello,

some days ago I had issues with xgettext.  I kept calling it for my C
project, but it would just say nothing and generate no file.  After some
search, I figured out the reason: I didn't pass the -k_ option, so the
program could not find any invocation of gettext() through the _ macro.

So I have three proposals:

- xgettext should say something when it can't find anything and it
  doesn't generate any file, because it's a puzzling behavior to
  beginners

- the _ alias for gettext() should be default for the C language, being
  also recommended in gettext documentation

- the xgettext docs describe all options, but should provide common usage
  examples, which of course include the -k_ option.


Let me know, I can work on all three. Warmly,
Andrea Monaco