wget: fix build against uclibc snapshot

Same fix as a728e2fe (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/bc20297dad0f0e9b7fa79fe835b9754fbce6dfdf/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>