Remove gcc version check. (#647)

* Remove gcc version check.

Use __cplusplus instead.

* Enable updated __cplusplus macro in MSVC

* Add a provision for MSVC 2015

* Don't use __cplusplus at all when MSVC is used

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

Revert "[CI] Reorder #include to comply with cpplint 1.5.0 (#622)" (#624

)

This reverts commit 88abbf9.

Added .editorconfig (#431)