Require -fPIC instead of just -fPIE for -reduce-relocations

GCC 5 combined with a recent binutils have a new optimization that
allows them to generate copy relocations even in -fPIE code. Clang has
the same functionality when compiling an executable with -flto. We need
to let the compilers know that they cannot use copy relocations, so they
need to use really position-independent code.

Position independent code throughout is not really required. We just
need the compilers to use position-independent access to symbols coming
from the Qt libraries, but there's currently no other way of doing that.

Task-number: QTBUG-45755
Change-Id: I0d4913955e3745b69672ffff13db5df7377398c5
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>