Silence _BSD_SOURCE warnings

glibc 2.20 requires _DEFAULT_SOURCE and doesn't like _BSD_SOURCE alone

Also see:
 - https://github.com/antirez/redis/pull/2189
 - https://sourceware.org/glibc/wiki/Release/2.20#Deprecation_of__BSD_SOURCE_and__SVID_SOURCE_feature_macros

Thanks to badboy for pointing out the problem at
https://github.com/redis/hiredis/issues/288#issuecomment-68849454

fmacros.h

@@ -1,8 +1,9 @@
 #ifndef __HIREDIS_FMACRO_H
 #define __HIREDIS_FMACRO_H
 
-#if !defined(_BSD_SOURCE)
+#if defined(__linux__)
 #define _BSD_SOURCE
+#define _DEFAULT_SOURCE
 #endif
 
 #if defined(__sun__)