[Patch] Document __secure_getenv

[Rich Felker]

On Thu, Jul 12, 2012 at 10:38:24AM -0700, Roland McGrath wrote:
> The public symbol needs a proper public declaration, not one in include/.
> We should also deprecate public use of __secure_getenv and make it
> unlinkable-to, as Carlos mentioned.

How can a symbol be made unlinkable, i.e. usable by already-linked
apps, but failing if ld tries to link a new app to it? This is
slightly off-topic but it's a problem I've been trying to solve for a
while and it looks like you might know the answe.

Rich