configure: restore -mdynamic-no-pic on darwin (missing since r20497)

Darwin compilers default to PIC on, so -mdynamic-no-pic (which disables it
again) needs to be added to cflags before enable pic is checked.

Fixes compilation without --enable-shared at least the 10.6 system compiler
for x86-32.
Fixes speed regression on all darwin arches except x86-64.

Originally committed as revision 22730 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2121,7 +2121,6 @@ esac
 
 enable $subarch
 enabled spic && enable pic
-check_cpp_condition stdlib.h "defined(__PIC__) || defined(__pic__) || defined(PIC)" && enable pic
 
 # OS specific
 case $target_os in
@@ -2292,6 +2291,8 @@ case $target_os in
         ;;
 esac
 
+check_cpp_condition stdlib.h "defined(__PIC__) || defined(__pic__) || defined(PIC)" && enable pic
+
 set_default $PATHS_LIST
 
 add_extralibs $osextralibs