-rdynamic
Pass the flag ‘-export-dynamic’ to the ELF linker, on targets that support
it. This instructs the linker to add all symbols, not only used ones, to the
dynamic symbol table. This option is needed for some uses of dlopen or to
allow obtaining backtraces from within a program.
rdynamic传递-export-dynamic给ELF linker。指示将所有的符号放到dynamic symbol table，这对于使用dlopen的方式使用的符号很有帮助。
--as-needed
--no-as-needed
This option affects ELF DT NEEDED tags for dynamic libraries mentioned
on the command line after the ‘--as-needed’ option.
这是ld的选项，控制一个命令行中的lib是否放入DT_NEEDED，前者没如果lib没有被用到，则不放入。后者始终方式，不同的ld默认行为不一样。