CDRIVER-4673 fix build of libmongoc using system libbson

[kevinAlbs]

Summary

• Move bson-dsl.h to src/common.
• Do not define mongo::detail::c_platform if using system libbson.
• Add regression test to ensure libmongoc can build with system install of libbson.

Background & Motivation

This PR resolves two observed issues when building libmongoc using a system install of libbson.

On the r1.24 branch, a compile error referenced in CDRIVER-4673 occurs: fatal error: 'bson/bson-dsl.h' file not found. bson-dsl.h is not installed with libbson, but bson-dsl.h is needed to compile libmongoc. 16d6237 moves bson-dsl.h to src/common. This is applied to r1.24 in #1388

On master, there is an additional error reported in cmake. Running ./.evergreen/scripts/install-libmongoc-after-libbson.sh results in:

-- Searching for libbson CMake packages
CMake Error at install-dir/lib/cmake/bson-1.0/bson-targets.cmake:42 (message):
  Some (but not all) targets in this export set were already defined.

  Targets Defined: mongo::detail::c_platform

  Targets not yet defined: mongo::bson_shared, mongo::bson_static

Call Stack (most recent call first):
  install-dir/lib/cmake/bson-1.0/bson-1.0-config.cmake:4 (include)
  CMakeLists.txt:329 (find_package)

8883017 will not define c_platform if using a system libbson install to fix this error.

[vector-of-bool]

LGTM!