folly: always enable PIE in shared libs build
folly: always enable PIE in shared libs build

Summary:
This reverts https://github.com/facebook/folly/pull/833 by adding back
the unconditional setting of `POSITION_INDEPENDENT_CODE` when building shared
libs. Without it, the build fails with a number of relocation errors. This was
previously reported in https://github.com/facebook/folly/issues/983

Differential Revision: D24449731

fbshipit-source-id: 6f6cb71ef545d02998afb09909772b62657c1262

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly