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
Fixes speed regression on all darwin arches except x86-64.
Originally committed as revision 22730 to svn://svn.ffmpeg.org/ffmpeg/trunk