[Rd] SystemRequirements clarification

[Paul Roebuck]

Did google search and checked R-ext but found very little
on this DESCRIPTION field. Does it document runtime dependency
only? If I have some code that can only be used with a specific
compiler (code makes use of GCC attributes), should I add
	SystemRequirements: GCC
to the package's DESCRIPTION to document the build-only
requirement.

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SIGSIG -- signature too long (core dumped)