Enable `-fPIC` for C library
Enable `-fPIC` for C library

Summary:
At least or x86_64 linux, it appears that you need position-independent
code is required in order to link against it. Otherwise you see errors
like this:

```
/usr/bin/ld: lib_sys.rlib(CSSLayout.c.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: lib_sys.rlib(CSSNodeList.c.o): relocation R_X86_64_32 against `.rodata.str1.8' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Nonrepresentable section on output
collect2: error: ld returned 1 exit status
```

If I understand it correctly, you don't strictly need PIC for static libraries, but as soon as you want to link *other* dynamically linked library into it, it is required - which makes logical sense to me. Let me know if you want to enable this by default or leave it up to the developers to enable this if needed.
Closes https://github.com/facebook/css-layout/pull/263

Reviewed By: passy

Differential Revision: D4237009

Pulled By: emilsjolander

fbshipit-source-id: e73ea0ea22520758ec958a031d6e2ca62fdcda15

facebook/css-layout,rmarinho/yoga,facebook/css-layout,yihuang/css-layout,yihuang/css-layout,facebook/yoga,rmarinho/yoga,rmarinho/yoga,yihuang/css-layout,facebook/css-layout,rmarinho/yoga,facebook/css-layout,rmarinho/yoga,facebook/css-layout,rmarinho/yoga,yihuang/css-layout,yihuang/css-layout,rmarinho/yoga,yihuang/css-layout,facebook/yoga,facebook/yoga,yihuang/css-layout,facebook/yoga,facebook/yoga,rmarinho/yoga,facebook/yoga,facebook/css-layout,rmarinho/yoga,yihuang/css-layout,facebook/yoga,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,rmarinho/yoga,facebook/yoga,yihuang/css-layout,facebook/css-layout,yihuang/css-layout