Detect whether $CXX is clang with gcc alias.

Use -exported_symbols_list under Clang/LLVM env

On macOS under llvm/clang++, the default delimiter for format string is
weird: it cannot read full CJK characters from the table files.

This patch adds macOS-specified code to make "\t" and " " the implicit
delimiter.

Ref:
https://stackoverflow.com/questions/12885628/changing-the-scanf-delimiter

* zhuyin_get_zhuyin_key
* zhuyin_get_zhuyin_key_rest