make ninja build with -std=c++17

Ninja is supposed to be able to build as C++98 so it can run on old
systems, but it should also be possible to optionally build it with
newer dialects.

(cherry picked from commit dfed28c3073947cf6b77a53a6bab06285ff04a11)

Change-Id: Id65cde8859eae0cb0b8905cba070f7f043e23163
2 files changed