CXX-3067 test for existing examples target before add_custom_target

[eramongodb]

Resolves CXX-3067. Companion to mongodb/mongo-c-driver#1675.

Avoid the C Driver imported via add_subdirectory(), which defines the examples target, from conflicting with CXX Driver's attempt to create an examples target. Otherwise the following error may be observed:

CMake Error at examples/CMakeLists.txt:33 (add_custom_target):
  add_custom_target cannot create target "examples" because another target
  with the same name already exists.  The existing target is a custom target
  created in source directory
  "_deps/mongo-c-driver-src/src/libbson".
  See documentation for policy CMP0002 for more details.

If a tests target is ever added to the CXX Driver, it will need the existence check as well.

[eramongodb]

No longer necessary due to prefixed target names by C Driver: mongodb/mongo-c-driver#1675 (comment)