This is needed to compile zlib support. I think it is by accident available in the original buildroot, but not in mine.
I think it is better to explicitly name it.
During update, RPM cannot simply replace directory by link to direcotry,
therefore create the directory structure and link the files there one by
one (rhbz#1691039).
This avoids issues running `$ bin/rails console --sandbox`, which
otherwise breaks with error such as:
~~~
$ bin/rails console --sandbox
Traceback (most recent call last):
12: from bin/rails:4:in `<main>'
11: from bin/rails:4:in `require'
10: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/commands.rb:18:in `<top (required)>'
9: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command.rb:44:in `invoke'
8: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command.rb:70:in `find_by_namespace'
7: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command/behavior.rb:79:in `lookup'
6: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command/behavior.rb:79:in `each'
5: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command/behavior.rb:80:in `block in lookup'
4: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command/behavior.rb:80:in `each'
3: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command/behavior.rb:84:in `block (2 levels) in lookup'
2: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/command/behavior.rb:84:in `require'
1: from /builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/commands/console/console_command.rb:3:in `<top (required)>'
/builddir/.gem/ruby/gems/railties-5.2.2.1/lib/rails/commands/console/console_command.rb:3:in `require': cannot load such file -- irb (LoadError)
~~~
The macros need to be udpated, since RPM 4.14+ can expand .gem files and
also also provide %{gem_name}.gemspec file used to rebuild the gem.
However, the .gemspec file is not in current directory, but directly in
%{_builddir}.
