[libc++] Do not force building with -fPIC (re-applying)

[ldionne]

Summary

In r355746, we stopped forcing to build with -fPIC because that should
be specified by the CMAKE_POSITION_INDEPENDENT_CODE option at CMake
configure time (and by default -fPIC is used for shared libraries anyways).

However, r355746 had to be reverted in r355756 because we were not
actually building the shared library with -fPIC. The reason is that
we were sharing an object library between the static and the shared
library, which caused flags for static libraries to be used when
building object files that were going to be used for a shared library.

Since this is resolved, we can stop forcing -fPIC again.

[ldionne]

@EricWF Any objection to re-shipping this since https://reviews.llvm.org/D59248 has landed?

[EricWF]

Go nuts. I'm assuming it should work now :-D

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()