coreutils: fix build against uclibc snapshot

uClibc development version adds support for POSIX spawn routines. However,
unlike glibc these routines are in librt. This breaks gnulib autoconf
detection. Teach gnulib autoconf to look for POSIX spawn in librt.

Fixes:
http://autobuild.buildroot.net/results/246/246b3778a1a646afd1c8b9c17b4579fb5a27120e/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>