Build: spec: make %license work even w/ older RPM versions

...not supporting that per se nor if it's worked around by distro-wide
RPM macros.(like EPEL6 does). This makes the RHEL6 build pass.

It was argued that Lua scripts support is not a guaranteed feature of RPM
and depends on how it is built. However, there's something to consider:

• RPM coexists with Lua for 12+ years
• Lua scripts are part of RPM's standard macros file for 8+ years
• default is to compile RPM with Lua support
• newer RPM-backed distros started using Lua-based standard convenience macros like %autosetup, so there's effectively no way back from Lua unless enormous effort is to be spent on the affected packages

and, furthermore:

• upstream's specfile is a reference (that tries to "cater all" on best effort basis), not something to be followed rigourously by various downstreams
• sustainable clarity > over-conditionalization
• the solution is hacky, by keep in mind RPM applies a similar hack internally when supporting %license properly as the authors have fallen into their own trap with "License:" tag + pre-existing mechanism vs. %license