Package updates for RHEL 7

Spec file and build changes for RHEL 7 build
Externalize Ant dependencies for different RHEL versions.
Replace %define macro with %global.
See
https://fedoraproject.org/wiki/Packaging:Guidelines#.25global_preferred_over_.25define
Switch from RPM_BUILD_ROOT to %buildroot. It's shorter.
Merge pull request #570 from candlepin/awood/package-el7

Package updates for RHEL 6/7