Sync to ruby/bigdecimal@92356ba71c6bd325b0ab618c634a7aecf8cdc767

* https://github.com/ruby/bigdecimal/compare/v1.4.0.pre.20181214a..v1.4.0.pre.20181220a

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66473 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* https://github.com/ruby/bigdecimal/compare/74d25ef..v1.4.0.pre.20181205a

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66222 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* Revert "Remove unnecessary linker flags"

  This reverts commit 49efa01579272d52d28bd361c498d96ca92e986e.

* Revert "Move dependency on gemspec"

  This reverts commit bfb64d476578eadd61a738149726da37840f068d.

* Revert "Remove unnecessary directory listing"

  This reverts commit 86661b5c60b8da6e5641c9c772b57857a1f988f4.

* Revert "* expand tabs."

  This reverts commit 35ada33f8386b56611f10b1a0a4e5667e9b08071.

* Revert "Import bigdecimal-1.4.0.pre-20181204a"

  This reverts commit 8891bb3bd602bdbabd24260cf1e431475dc027c8.

* Revert "ext/bigdecimal/bigdecimal.c: drop unused function"

  This reverts commit 5ceeea4da1179193c2c7ddcebfd67019128473e3.

* Revert "* expand tabs."

  This reverts commit e021386e905b6d6799a84dfbc2f0592e42626366.

* Revert "Import bigdecimal-1.4.0.pre-20181130a"

  This reverts commit a0e438cd3c28d2eaf4efa18243d5b6edafa14d88.

* Revert "NEWS: write about bigdecimal updates"

  This reverts commit 89455ff2ba973f2a9ee7b79657e0e6f1f31501f9.

* Revert "Fix rubyspec of bigdecimal for ruby <2.6"

  This reverts commit 4bcdeeb65e57de4da306d347652898c767162e74.

* Revert "Fix rubyspec against bigdecimal updates"

  This reverts commit c8fb30fc9e0ee14e87be1e231869a12aaef1eedf.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66210 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Cygwin/mingw linker should be able to link against shared library
itself.  Mswin build sets -def:$(DEFFILE) option by the default.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66194 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* https://github.com/ruby/bigdecimal/compare/74d25ef..v1.4.0.pre.20181130a

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66124 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* ext/bigdecimal/extconf.rb: extconf.h depends on the gemspec
  file.  [ruby-core:84247] [Bug #14180]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61221 b2dd03c8-39d4-4d8f-98ff-823fe69b080e