Add -D_POSIX_C_SOURCE=200112 to cflags.

Originally committed as revision 14754 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -974,7 +974,7 @@ enable stripping
 vhook="default"
 
 # build settings
-add_cflags -D_ISOC99_SOURCE
+add_cflags -D_ISOC99_SOURCE -D_POSIX_C_SOURCE=200112
 SHFLAGS='-shared -Wl,-soname,$$(@F)'
 VHOOKSHFLAGS='$(SHFLAGS)'
 FFSERVERLDFLAGS=-Wl,-E