MONGOCRYPT-839 address CMP0169 for CMake 3.30 and newer

[eramongodb]

Followup to #1052.

Addresses CMP0169, which has been emitting a warning since CMake 3.18, but is made an error since 3.30 when the policy is set to NEW (as is now the case following #1052):

CMake Error at FetchContent.cmake:1951 (message):
  Calling FetchContent_Populate(embedded_mcd) is deprecated, call
  FetchContent_MakeAvailable(embedded_mcd) instead.  Policy CMP0169 can be
  set to OLD to allow FetchContent_Populate(embedded_mcd) to be called
  directly for now, but the ability to call it with declared details will be
  removed completely in a future version.
Call Stack (most recent call first):
  cmake/FetchMongoC.cmake:35 (FetchContent_Populate)
  cmake/ImportBSON.cmake:105 (include)
  CMakeLists.txt:27 (include)

This is the same issue described in mongodb/mongo-cxx-driver#1186, and the solution is the same: using *_MakeAvailable() for CMake 3.18 and newer (which supports SOURCE_SUBDIR), and using *_Populate() for backward compatibility until the minimum required CMake version is raised to 3.18 or newer. This applies to IntelDFP (targets manually defined in IntelDFP.cmake) and libbson (manual call to add_subdirectory() in ImportBSON.cmake).

Interestingly, the initial attempt to apply this change produced the following error:

CMake Error at MongoPlatform.cmake:19 (add_library):
  add_library cannot create target "_mongo-platform" because another target
  with the same name already exists.  The existing target is an interface
  library created in source directory
  "_deps/embedded_mcd-src".
  See documentation for policy CMP0002 for more details.
Call Stack (most recent call first):
  _deps/embedded_mcd-src/CMakeLists.txt:29 (include)

This is due to the mongo-c-driver project being add_subdirectory()'d twice: once by *_MakeAvailable() and once again later in ImportBSON.cmake. Unlike in mongodb/mongo-cxx-driver#1186, it seems newer CMake versions do not accept SOURCE_SUBDIR "" as a workaround (possibly due to CMP0174?). Therefore, although a bit more awkward, SOURCE_SUBDIR is set to "/dev/null" instead of "". (It just needs to be a (non-existent) directory which does not contain a CMakeLists.txt file.)

[eramongodb]

After further testing, SOURCE_SUBDIR "/dev/null" is rejected by CMake 3.15:

CMake Error at ExternalProject.cmake:1668 (message):
  External project embedded_mcd-populate has non-relative SOURCE_SUBDIR!

Replaced /dev/null with a reserved NO_ADD_SUBDIRECTORY "directory" (none of the FetchContent projects we import should have a directory with this name or a CMakeLists.txt file within it).