v3/ec: enable C++17

Pigweed requires C++17 to build, so add the necessary config values
and the `extern "C"` so that all of the C code can compile for use
in C++ programs.

BUG=None
TEST=run ad-hoc tests in doc/README.md

Change-Id: I4e16784cf9fc967210299946e772d5f540f6156d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/chameleon/+/3179861
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Alexandru M Stan <amstan@chromium.org>
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
10 files changed