Fixes #5651: Use the same SPEC macro version comparison everywhere

Ticket: http://www.rudder-project.org/redmine/issues/5651

To be merged in master

This commit uses the same way of comparing OS versions everywhere, to prevent "less than / less of equal than" comparisons from breaking due to macro expansion to 0 if the macro is not defined.

0%{?rhel} <= 4 becomes %if 0%{?rhel} && 0%{?rhel} <= 4 (making the expansion to 0 harmless as the first condition is also 0, meaning false in the case of a "if"
I'm a bit concerned about changing the name of the macro we use here. I know that RHEL 5, and in particular CentOS 5 have had issues with availability of the macros. Are you sure these are available on our builders?