Removed ifdef from library path construction.
Removed ifdef from library path construction.

We don't need to use ifdefs to construct the platform-specific
Mesos library name, we can use the stout `os::libraries` API
instead.

Review: https://reviews.apache.org/r/67736/

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