Clang warns me,
clang: warning: argument unused during compilation: '-Os'
During compile with make. Why it doesn't accept the optimization flag?
-Os isn't a valid optimization option for (most) clang - it's not completely flag-compatible with GCC.
The clang man page is authoritative on what options your particular version accepts (specifically, optimizing by size is not yet a primary concern for LLVM). Apple has pushed back patches for -Os and -Oz, but most packages don't include these updates.