wrong cmake path include

[Nana]

cmake throw an error when add_subdirectory, i check the CMakeLists.txt, here is what it shuod be
in CMakeLists.txt 51
it shoud be
include(${CMAKE_CURRENT_SOURCE_DIR}/cmake/CheckAtomic.cmake)
not
include(${CMAKE_SOURCE_DIR}/cmake/CheckAtomic.cmake)

[Tsung-Wei Huang]

@Anesthesia4 thanks for the catch! Could you please help submit a pull request? Thanks!