Fixed detection of ELFIO header in libprocess.
Fixed detection of ELFIO header in libprocess.

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

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