Fixed detection of ELFIO header in Stout.
Fixed detection of ELFIO header in Stout.

Build fails to detect system provided ELFIO headers when using `configure --with-elfio` flag. Basically this is the same fix as the patch
provided here: https://issues.apache.org/jira/browse/MESOS-6605

This closes #332

Gilbert88/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,kaysoky/mesos,jpeach/mesos,kaysoky/mesos,jpeach/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,Gilbert88/mesos,gsantovena/mesos,chhsia0/mesos,abudnik/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,gsantovena/mesos,gsantovena/mesos,reneploetz/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,abudnik/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,Gilbert88/mesos