Add support for CMake submodule

[h3x4n1um]

Allow using fifo_map as a submodule in CMake

Example:

# use fifo_map submodule
add_subdirectory(third_party/fifo_map)

# link fifo_map submodule
target_link_libraries(foo PRIVATE fifo_map::fifo_map)

[coveralls]

Coverage remained the same at 100.0% when pulling fd37ab542d9bbfe6224b3734261b8d82737ed162 on h3x4n1um:cmake-submodule into a5a5b4dc92b0031ec30b37b6c29944d450054df5 on nlohmann:master.

[nlohmann]

Thanks!