CDRIVER-4673 fix build of libmongoc using system libbson (#1387)

* move bson-dsl.h to src/common

bson-dsl.h is not installed, but shared between libmongoc and libbson.
Intended to resolve CDRIVER-4673.

* do not define `mongo::detail::c_platform` if using system libbson

* add regression test

* fix more includes

* add include of common-prelude.h

* fix more include paths