Check if PIC is somehow already enabled (e.g. a compiler that enables it by default).
This in particular ensures that -DPIC is set appropriately for YASM, which avoids
linking failures on x86_64 for compilers that have PIE enabled by default
(e.g. hardened Gentoo).
It also makes the macho64 special-case hack adding -DPIC to YASMFLAGS obsolete.
Originally committed as revision 20497 to svn://svn.ffmpeg.org/ffmpeg/trunk