The header checks described in CDRIVER-2797 are repeated throughout the codebase.

We should consider refactoring them into three files

mongoc-header-check.h, consisting of:

#if !defined(MONGOC_INSIDE) && !defined(MONGOC_COMPILATION)
#error "Only <mongoc.h> can be included directly."
#endif

bson-header-check.h, consisting of:

#if !defined(BSON_INSIDE) && !defined(BSON_COMPILATION)
#error "Only <bson/bson.h> can be included directly."
#endif

And common-header-check.h, consisting of:

#if !defined(MONGOC_INSIDE) && !defined(MONGOC_COMPILATION) && \
!defined(BSON_INSIDE)	 !defined(BSON_COMPILATION)
#error "Only <mongoc/mongoc.h> or <bson/bson.h> can be included directly."
#endif

We can verify with an Evergreen task that each header (except bson.h and mongoc.h) includes one of these checks directly after the license. We could also use the same check to verify that each file has a license.