vm: build with -fPIC to simplify linking

[ygrek]

No description provided.

[pchaigno]

What's the use case for this change?

[pchaigno]

@ygrek Could you detail the use case for this change?

[coveralls]

Coverage remained the same at 96.68% when pulling 7ca98a6 on ygrek:pic into 10e0a45 on iovisor:master.

[ygrek]

Shared objects require PIC, so code without -fPIC can not be linked into another .so
In my case (ocaml bindings in bytecode mode) all C code is compiled into shared object and then loaded by ocaml interpreter. So without -fPIC it is not easily possible to use ubpf from bytecode executables which is quite inconvenient.
I personally think that -fPIC should be used by default in libraries unless there are special performance considerations.
Naturally -fPIC changes code emitted by gcc into somewhat less efficient at function call sites, but difference is generally considered negligible. Though thinking about it ubpf interpreter is call-heavy probably so might suffer more. But anybody looking for perfomance will want to use jit so that should not matter?

[ygrek]

Example usage : https://github.com/ahrefs/ocaml-ubpf

[Alan-Jowett]

@ygrek it looks like this option was added in another PR. Are you ok closing this PR?

[Alan-Jowett]

Closing this PR as it appears to have been superseded by another PR.