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

[ldionne]

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.

[smeenai]

LGTM

libcxx/lib/CMakeLists.txt

if (LIBCXXABI_USE_LLVM_UNWINDER)
   elseif (LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))
     add_interface_library(unwind_static)
   else()
     add_interface_library(unwind)
   endif()
 endif()
 
 # Setup flags.
-if (NOT WIN32)
-  add_flags_if_supported(-fPIC)
-endif()
-
 add_link_flags_if_supported(-nodefaultlibs)
 
 if (LIBCXX_TARGETING_MSVC)
   if (LIBCXX_DEBUG_BUILD)
     set(LIB_SUFFIX "d")
   else()
     set(LIB_SUFFIX "")
   endif()