Use -fpic for dynamic builds, not -fPIC for static

This is what's meant to be used for ELF binaries, according to part 2
"Optimizations for DSOs" of Ulrich Drepper's "How to Write Shared
Libraries".

Also, it should quiet down GCC on Windows rightfully complaining that
-fpic/-fPIC doesn't do anything for static libraries, from previous
experience of compiling NitroHack on Windows.