write_compiler_detection_header writes a relative path
write_compiler_detection_header writes what looks like a full path into the build directory.
for example what should be /home/ian/path/to/cpp-proj/build/${OUTPUT_DIR} is written as /home/ian/path/to/cpp-proj/build/home/ian/path/to/cpp-proj/build/${OUTPUT_DIR}
the variable OUTPUT_FILES_VAR reports the file has been written to /home/ian/path/to/cpp-proj/build/${OUTPUT_DIR}
This is on CMake 3.10.1–1 on archlinux (cmake came from this pacman repo)