Drop -fpic; it hurts performance and we don't build libsparse.so by default

Drop -fpic; it hurts performance and we don't build libsparse.so by default

Sparse doesn't have a stable shared library interface, and doesn't build
libsparse.so by default.  However, it used -fpic, which incurred a noticeable
performance hit.  Remove -fpic from the default CFLAGS, and add a comment near
the definition of LIBS about adding it back if building a shared library.  You
probably want to build Sparse twice if you want a shared library: once without
-fpic to build the Sparse tools, and again with -fpic to build the shared
library.

Signed-off-by: Josh Triplett <josh@freedesktop.org>