* Use gnu for ruby build because strptime is provided by posix

* Move option to extconf.rb

* Remove unused code in Rakefile

* Add config files for kokoro test

* Use gnu 11

* Define _XOPEN_SOURCE

* Add gnu11 option

* Remove XOPEN

* Try base_cc_flags

* Try config_options

* Move time.h to top

* Try -D_XOPEN_SOURCE=700

* Define XOPEN_SOURCE directly

* Try extconf.rb

* Try CFLAGS

* Try ext.cross_config_options

* Make mac ruby release job fail on error

* Try again

* Try define _XOPEN_SOURCE in extconf.rb

* Try again

* Define __USE_XOPEN

* Remove 2.6.0

* No mingw

* Throw error on mingw

* Remove XOPEN_SOURCE in upb

* Add back mingw

* Remove comment

* Install rake compiler

* Add kokoro config to build ruby gem on linux

* Rename from linix to linux

* Fix prepare_build.sh name

* Clean up

* Install bundler

* Install bundler

* Use c99 in order to build gem on mingw-32 on ruby 2.0.0

See https://github.com/rake-compiler/rake-compiler-dock/issues/4

* Move c99 config to extcofig.rb