Link failure with ld.gold, -Wl,--as-needed and --enable-libaria2

[mgorny]

When aria2 is built with --enable-libaria2 most of the relevant code is built into the shared library. However, src/Makefile.am puts almost all linked dependency libraries into LDADD which applies to executables only. Therefore, most of the NEEDED entries land in the executable rather than the library.

This has two consequences:

1. The library lacks proper dependencies and therefore can't be used by a program unless it links to all its dependency libraries directly.
2. If aria2c is linked using ld.gold (the new binutils linker) with -Wl,--as-needed, the linker detects that aria2c executable does not use most of the passed -l libraries and removes them. As a result, aria2c is broken since neither the program nor the library links to necessary dependency libraries.

I think the correct solution here is to move all the dependency libraries from LDADD to libaria2_la_LIBADD.

[tatsuhiro-t]

Thank you. I made a commit as you suggested. See 3220989