The 3.10.0 release added the ENABLE_ABI_TAG_IN_PKGCONFIG_FILENAMES CMake configuration variable and updated pkg-config file generation scripts accordingly, but did not account for the fact that pkg-config dependencies depend on the pkg-config filename, not the library name.
The bsoncxx_name should be set to BSONCXX_OUTPUT_NAME (+ -static suffix as needed) when ENABLE_ABI_TAG_IN_PKGCONFIG_FILENAMES=OFF (the default).