Motivation
Ensure that libmongoc and libbson headers included in C++ projects to ensure C++ consumers will continue to compile correctly.

CDRIVER-4158 notes a regression that only applies to MSVC.

Scope
Add a task to test to compile a simple executable with a C++ compiler. Include all libbson and libmongoc headers. It should be sufficient to include "mongoc/mongoc.h".

The task should run on variants to cover major compilers. It may be sufficient to test a reasonable subset of g+, clang+, and MSVC on all variants on evergreen.