Introduce the "pic" / "no-pic" config option

Building shared libraries or not is not the same as building position
independent code or not.  It's true that if you don't build PIC, you
can't build shared libraries.  However, you may very well want to
build only static libraries but still want PIC code.

Therefore, we introduce a new configuration option "pic", which is
enabled by default or explicitely with "enable-pic", or disabled with
"no-pic" or "disable-pic".  Of course, if "pic" is disabled, "shared"
and "dynamic-engine" are automatically disabled as well.

Reviewed-by: Rich Salz <rsalz@openssl.org>