using dynamic lib default in linux-x86_64

[rayowang]

Like wasmer, use dynamic library by default in linux-x86_64 to prevent duplicate symbol. (#150)

[alexcrichton]

Thanks for the PR, but I'm hesitant to switch the defaults here. How does that work with distribution of the dynamic library itself? Do all users have to specify LD_LIBRARY_PATH as you've done here? That's what makes me think that while this is a good option to have it's not a better default just yet.

[scottopell]

I believe with shared libraries you do have the additional overhead of managing LD_LIBRARY_PATH or similar. This mentions being inspired by wasmer, and you can see in this wasmer PR that using a dynamic lib can cause some pain in distributing the resulting go binaries.