Always add -DPIC to YASM flags when pic is enabled.

This covers the previous case of x86_64 and shared, but also
works with --enable-pic.
For 32 bit x86 it currently makes no difference since x86inc.asm
ignores -DPIC for non-x86_64.

Originally committed as revision 20476 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2231,7 +2231,7 @@ EOF
 
     YASMFLAGS="-f $objformat -DARCH_$(toupper $subarch)"
     enabled     x86_64        && append YASMFLAGS "-m amd64"
-    enabled_all x86_64 shared && append YASMFLAGS "-DPIC"
+    enabled     pic           && append YASMFLAGS "-DPIC"
     case "$objformat" in
         elf) enabled debug && append YASMFLAGS "-g dwarf2" ;;
         macho64)              append YASMFLAGS "-DPIC -DPREFIX" ;;