Use Texinfo macros for "GNU C Library" references in manual

[Roland McGrath]

I didn't actually vet all the text changes.  But assuming no typos, etc.
this looks good to me.

wrt NOTES, originally it was included as a README-like file because the
concept of feature test macros was new and unfamiliar and so it was a
significant thing to mention for using libc.  Nowadays I don't think there
is a particular need to call attention to it any more than the rest of the
documentation.  So I think it would be fine to drop the file.


Thanks,
Roland