build: Fix check for 'syntax' ruby gemHEADmaster

With `ruby` pointing to ruby3.1 and `syntax` only installed with `RUBY_ABIS: 3.0`
configure succeeded, but the build failed trying to build the Ruby API docs:
```
<internal:/usr/x86_64-pc-linux-gnu/lib/ruby/3.1/rubygems/core_ext/kernel_require.rb>:85:in `require': cannot load such file -- syntax/convertors/html (LoadError)
        from <internal:/usr/x86_64-pc-linux-gnu/lib/ruby/3.1/rubygems/core_ext/kernel_require.rb>:85:in `require'
        from /var/tmp/paludis/build/sys-apps-paludis-scm/work/paludis-scm/doc/api/ruby/create_ruby_examples.rb:5:in `<main>'
make[2]: *** [doc/api/ruby/CMakeFiles/ruby-docs.dir/build.make:77: doc/api/ruby/ruby] Error 1
```

The configure check didn't catch this because:
 - it used `OUTPUT_VARIABLE` instead of `RESULT_VARIABLE`
 - it used the `-r` with a wrong argument

The check wrongly invoked `ruby` with `-rubygems` instead of `-r rubygems`:
```
$ ruby3.0 -rubygems -e "require 'syntax/convertors/html'"
<internal:/usr/x86_64-pc-linux-gnu/lib/ruby/3.0/rubygems/core_ext/kernel_require.rb>:85:in `require': cannot load such file -- ubygems (LoadError)
Did you mean?  rubygems
        from <internal:/usr/x86_64-pc-linux-gnu/lib/ruby/3.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
```

Fix the ruby invocation and check if the exit code (`RESULT_VARIABLE`) is non-zero

Fixes: a8c9e5d64 ("build: introduce cmake based build system")

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