libm is used with mozjpeg and built libjpeg, but not if using system's,
yet jpegoptim.c uses floor() either way.
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -356,3 +356,3 @@
         target_include_directories(${PROJECT_NAME} PRIVATE ${JPEG_INCLUDE_DIRS})
-        target_link_libraries(${PROJECT_NAME} JPEG::JPEG)
+        target_link_libraries(${PROJECT_NAME} JPEG::JPEG m)
     endif()