Use check_cflags for -mdynamic-no-pic so third-party compilers will work on Darwin.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@13007 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -1358,7 +1358,7 @@ if test $target_os = darwin; then
         add_cflags "-pipe"
         check_cflags "-force_cpusubtype_ALL"
         check_cflags "-Wno-sign-compare"
-        enabled shared || add_cflags -mdynamic-no-pic
+        enabled shared || check_cflags -mdynamic-no-pic
     fi
 fi