touch and utimens troubles on new/old software combinations

[Mike Frysinger]

a recent gnulib commit (faeb3e6b21...) causes trouble for some packages (such 
as touch in coreutils) on certain combinations of software.  for example, if 
you're running a recent version of glibc (say 2.7) compiled against recent 
kernel headers (say 2.6.25) but execute on an older kernel (say 2.6.18), then 
the resulting touch binary will attempt to use utimensat() which fails with 
ENOSYS.

i'm not sure which route is best for gnulib: should it try each version at 
runtime and slowly fall back, or should it review the gnu/stubs.h header 
provided by glibc so that it knows ahead of time that the utimensat() symbol, 
while available in the C library, is not actually usable.
-mike

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