compilation fails on ruby <2.7

[ahorek]

the gem claims to support ruby 2.5+, but on ruby 2.5/2.6 it fails on missing dependencies

make: *** No rule to make target '/opt/hostedtoolcache/Ruby/2.6.8/x64/include/ruby-2.6.0/ruby/assert.h', needed by 'escape.o'.  
Stop.

actually, the gem doesn't compile extensions at all because there's a missing line in cgi.gemspec

spec.extensions    = ["ext/cgi/escape/extconf.rb"]

I think it's a bug or is it intentional?