add -fPIC to julia-config.jl

[mlubin]

@yuyichao found that -fPIC is needed so that the GC stack is properly initialized. I've tested on linux only.

[yuyichao]

Not just jl_pgcstack, but every global variables used by the main executable. Somehow without -fPIC the compiler creates static version of these variables in the executable. However, the linker doesn't seems to be consistent about which version to resolve to. libjulia.so, the executable and dlsym all resolve to the version in the executable but sys.so resolves to the one in libjulia.so.

[yuyichao]

Should probably update the embedding doc too.

[mlubin]

done

[tkelman]

-fPIC gives an annoying warning (and is the default) on windows. Is this needed on mac and 32 bit linux as well? If so it should be unix only.

[tkelman]

And if julia-config isn't tested, it probably should be so we don't break it (maybe subset of #8757 though)

[mlubin]

I'm not really in a position to add these tests, all I know is that without -fPIC on linux, the embedding code doesn't work.

[tkelman]

Needs to at least be unix only, and maybe more specific to just linux? Just 64 bit linux? Or any 64 bit unix? Depending on where it's required.

[mlubin]

Anyone have an idea if this is needed on OS X also?

[tkelman]

or freebsd for that matter?

[mlubin]

I doubt it's harmful though

[yuyichao]

We pass -fPIC on everywhere other than windows so we should do that for embedding code too.

[tkelman]

On Windows it's a really annoying and prominent warning, so absolutely needs to be taken out there.

[mlubin]

How's this?

[yuyichao]

LGTM