Scope

Disable shared libmongoc targets if the CMake option ENABLE_SHARED is OFF

Background and motivation

CDRIVER-4705 adds a ENABLE_SHARED option. If OFF, building shared libbson targets is disabled. This is not yet applied to libmongoc.

This may result simplifying the vcpkg port. The port currently includes a patch to disable the shared library build when the static library is built:
https://github.com/microsoft/vcpkg/blob/ac2a14f35fcd57d7a38f09af75dd5258e96dd6ac/ports/libbson/disable-dynamic-when-static.patch

IIUC vcpkg expects only shared or static libraries are built (but not both). Refer:
https://learn.microsoft.com/en-us/vcpkg/contributing/maintainer-guide#choose-either-static-or-shared-binaries