Windows: Added check for Admin privileges in build script.
Windows: Added check for Admin privileges in build script.

The Mesos agent in particular creates symlinks regularly.  On Windows,
symlink creation requires Administrator privileges, which means that
the Mesos test suite must be run as an Administrator.  The test suite
will generally CHECK-fail otherwise.

neilconway/mesos,Gilbert88/mesos,reneploetz/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,andschwa/mesos,gsantovena/mesos,verizonlabs/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,zmalik/mesos,Gilbert88/mesos,verizonlabs/mesos,andschwa/mesos,craimbert/mesos,Gilbert88/mesos,abudnik/mesos,neilconway/mesos,dforsyth/mesos,neilconway/mesos,craimbert/mesos,asamerh4/mesos,zmalik/mesos,neilconway/mesos,andschwa/mesos,asamerh4/mesos,zmalik/mesos,verizonlabs/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,jpeach/mesos,reneploetz/mesos,jpeach/mesos,jpeach/mesos,dforsyth/mesos,zmalik/mesos,craimbert/mesos,dforsyth/mesos,gsantovena/mesos,craimbert/mesos,asamerh4/mesos,dforsyth/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,chhsia0/mesos,gsantovena/mesos,kaysoky/mesos,yuquanshan/customizedMesos,craimbert/mesos,reneploetz/mesos,chhsia0/mesos,yuquanshan/customizedMesos,craimbert/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,kaysoky/mesos,chhsia0/mesos,gsantovena/mesos,neilconway/mesos,abudnik/mesos,asamerh4/mesos,chhsia0/mesos,yuquanshan/customizedMesos,reneploetz/mesos,Gilbert88/mesos,gsantovena/mesos,neilconway/mesos,kaysoky/mesos,Gilbert88/mesos,zmalik/mesos,gsantovena/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,verizonlabs/mesos,andschwa/mesos,asamerh4/mesos,andschwa/mesos,abudnik/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,jpeach/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,neilconway/mesos,andschwa/mesos,verizonlabs/mesos,jpeach/mesos,verizonlabs/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,dforsyth/mesos,zmalik/mesos,gsantovena/mesos,andschwa/mesos,Gilbert88/mesos,asamerh4/mesos,abudnik/mesos,asamerh4/mesos,craimbert/mesos,dforsyth/mesos,jpeach/mesos,chhsia0/mesos