i2c-tools: fix feature test macros for glibc >= 2.20

Since glibc 2.20, the usage of _BSD_SOURCE is deprecated. Fix it like
described here:

https://sourceware.org/glibc/wiki/Release/2.20#Deprecation_of__BSD_SOURCE_and__SVID_SOURCE_feature_macros

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

tools/i2cbusses.c

@@ -23,7 +23,8 @@
 */
 
 /* For strdup and snprintf */
-#define _BSD_SOURCE 1
+#define _BSD_SOURCE 1 /* for glibc <= 2.19 */
+#define _DEFAULT_SOURCE 1 /* for glibc >= 2.19 */
 
 #include <sys/types.h>
 #include <sys/stat.h>