ENG-5123: Fixed RPM build errors on CentOS 6.x

As of Centos 6.x (and Fedora 10+) the presence of the BuildRoot tag in the spec file is not required and if one is defined it will be ignored
Also the provided buildroot will automatically be cleaned before commands in %install are called, so it is insufficient to specify %_buildroot as the %_tmppath

See: http://fedoraproject.org/wiki/Packaging:Guidelines#BuildRoot_tag