libsensors: Prevent warning with 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

Inspired from a similar patch by Wolfram Sang for i2c-tools.

Signed-off-by: Jean Delvare <jdelvare@suse.de>

lib/init.c

@@ -20,7 +20,8 @@
 */
 
 /* Needed for scandir() and alphasort() */
-#define _BSD_SOURCE
+#define _BSD_SOURCE 1 /* for glibc < 2.19 */
+#define _DEFAULT_SOURCE 1 /* for glibc >= 2.19 */
 
 #include <sys/types.h>
 #include <sys/stat.h>