我在用cmake version 3.5.1編譯時，發現 set(LIBRARY_OUTPUT_DIRECTORY ${PROJECT_SOURCE_DIR}/lib) 不起作用，後來我把LIBRARY_OUTPUT_DIRECTORY改成 LIBRARY_OUTPUT_PATH就可以了。
cmake官網的解釋：
Old executable location variable.
The target property RUNTIME_OUTPUT_DIRECTORY supercedes this variable for a target if it is set. Executable targets are otherwise placed in this directory.
有誰清楚的，可以在評論區評論