remove additional -fPIC switch because

remove additional -fPIC switch because

- cmake is smart enough to add this flag on its own (for object files
  that it links into a shared library)
- cmake contains two sets of oject files - the position-dependent ones
  and the position-independent ones - and uses each set accordingly
  (for linking the executable, static library, shared library)
- having an executable that unnecessarily contains position-independent
  code has performance implications