set locales to C because en_US not installed on some systems, only UTF-8 version and en_US.UTF-8 does not work.

Added message of the day and made it properly configurable using /#setting (not saved to config file yet)

Made picking items up in creative mode delete them (because they usually are in the inventory already anyway)

Some more gettext stuff

Otherwise the USE_GETTEXT define might not be visible.

some more gettext fixes

Now the user-level option is called ENABLE_GETTEXT, and USE_GETTEXT is
only set to true if gettext was enabled and found. This simplifies all
check to USE_GETTEXT only rather than the double checks for it being
enabled and found.

This solves two issues at once:
* CMake would delete po files during ‘make clean’ because it thought
  they were autogenerated and not just managed
* the only gettext tools readily available in Windows are so old they
  don't support options like --package-name

The change also moves minetest.pot down one level, so we don't need to
special case ‘en’ anymore.

The downside is of course that you need some sane POSIX shell to update
the po files.

Put the gettext init code in a macro and define it appropriately
depending on whether gettext is actually available or not.