The dependency of extconf.h on bigdecimal.gemspec does not make sense
as far as no rule is defined for it.  Also, the relationship between
extension library and gemspec file is various in default gems, and
does not work well.

https://github.com/ruby/bigdecimal/commit/7f99b28552

```
$ gem build bigdecimal.gemspec
WARNING:  license value 'BSD-2-clause' is invalid.  Use a license identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
Did you mean 'BSD-2-Clause'?
WARNING:  See https://guides.rubygems.org/specification-reference/ for help
```

https://github.com/ruby/bigdecimal/commit/36b77a2d2f

JRuby currently ships its own internal bigdecimal extension as
part of the core libraries. In order for users to be able to add
bigdecimal to their Gemfile or gem dependencies, we need to stub
out the C extension and just load the extension shipped with
JRuby.

In the future we will try to move our BigDecimal implementation
into the gem, but for now this is the simplest way to make it
installable on JRuby.

See #169

https://github.com/ruby/bigdecimal/commit/829956c643

https://github.com/ruby/bigdecimal/commit/25a75c2033

https://github.com/ruby/bigdecimal/commit/13165b29b8

Co-authored-by: Hiroshi SHIBATA <hsbt@ruby-lang.org>

https://github.com/ruby/bigdecimal/commit/57e2194135

https://github.com/ruby/bigdecimal/commit/2ef67c2cc7

https://github.com/ruby/bigdecimal/commit/c3453d2b97

e.g.
```
gem "bigdecimal", github: "ruby/bigdecimal"
```

It would fail because bundler regenerates the `gemspec`,
so `bigdecimal_version` is gone.

https://github.com/ruby/bigdecimal/commit/bccaa66f2c