[libc++] Do not force building with -fPIC

Summary:
Whether we build with -fPIC should be specified by the
CMAKE_POSITION_INDEPENDENT_CODE option at configure time.
Note that this patch doesn't change the behavior when building
by default, since -fPIC is used for shared libraries by default.

Reviewers: EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59146

llvm-svn: 355746