[Rd] tcltk capability

[Gabor Grothendieck]

There are quite a few packages that make use of tcltk and although

- most R distributions have tcltk capability
- its possible to query this via capabilities()[["tcltk"]]
- attempting to build a package that needs it on such an R
distribution will give a message letting one know

users still seem to have problems with this.

There was recently an R bug report submitted that likely stemmed from
the user having such a build of R and recently there was also an R
help post that seemed to be related to this as well.  As more and more
packages add GUI capability these incidents are bound to increase.

The best situation would be to arrange that every distribution of R
have tcltk capability but if this is difficult to arrange then I
suggest there be a message at start up similar to the existing locale
message letting the user know that the R distribution they are running
lacks tcltk capability.  Preferably the message would be somewhat
menacing so that users are encouraged to get a different distribution
of R right from the start.

This might reduce such problem reports somewhat.

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