apr: fix threading lib detection

apr's configure doesn't try to guess which library is needed for
threading when cross-compiling hence this is never reflected in
apr-1-config, leading to a build failure such as:
http://autobuild.buildroot.net/results/035/035a00de0a56be00559767ba822a396ddc18a9fb/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>