Restore use of -fPIC for static builds again

This partly reverts commit 888a7c9c1cfe (-fPIC/-fpic change).  The flag
was originally set so that changing from a static build to a dynamic one
wouldn't require manually invoking 'make clean' in-between.

The flag is also changed back to -fPIC from -fpic, since it seems that
that's what CMake uses internally for shared libs, and I don't know how
well they'd mix.