Change _ISOC9X_SOURCE to _ISOC99_SOURCE.

Originally committed as revision 14739 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -974,7 +974,7 @@ enable stripping
 vhook="default"
 
 # build settings
-add_cflags '-D_ISOC9X_SOURCE'
+add_cflags '-D_ISOC99_SOURCE '
 SHFLAGS='-shared -Wl,-soname,$$(@F)'
 VHOOKSHFLAGS='$(SHFLAGS)'
 FFSERVERLDFLAGS=-Wl,-E