Use _DEFAULT_SOURCE to enable default libc features

Fix this warning from glibc:

/usr/include/features.h:183:3: warning: #warning "_BSD_SOURCE and
_SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Wcpp]

Keep _BSD_SOURCE for backward compatibility.

Makefile

 # multicat Makefile
 
 VERSION = 2.2
-CFLAGS += -Wall -Wformat-security -O3 -fomit-frame-pointer -D_FILE_OFFSET_BITS=64 -D_ISOC99_SOURCE -D_BSD_SOURCE
+CFLAGS += -Wall -Wformat-security -O3 -fomit-frame-pointer -D_FILE_OFFSET_BITS=64 -D_ISOC99_SOURCE -D_BSD_SOURCE -D_DEFAULT_SOURCE
 CFLAGS += -g
 # Comment out the following line for Mac OS X build
 LDLIBS += -lrt -pthread