all: Compile globally without _GNU_SOURCE, define _BSD_SOURCE or _GNU_SOURCE where needed

src/disk.c

@@ -16,6 +16,12 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#ifdef __linux__
+#ifndef _BSD_SOURCE
+#define _BSD_SOURCE
+#endif
+#endif
+
 #include <fcntl.h>
 #include <libgen.h>
 #include <unistd.h>