configure: be more liberal with nm output

Originally committed as revision 21324 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2223,7 +2223,7 @@ enabled pic && enable_pic
 check_cc <<EOF || die "Symbol mangling check failed."
 int ff_extern;
 EOF
-sym=$($nm -P -g $TMPO | grep ff_extern)
+sym=$($nm -g $TMPO | awk '/ff_extern/{ print substr($0, match($0, /[^[:blank:]]*ff_extern/)) }')
 extern_prefix=${sym%%ff_extern*}
 
 check_cc <<EOF && enable inline_asm