build: Use _DEFAULT_SOURCE.

_BSD_SOURCE is now deprecated in favor of _DEFAULT_SOURCE.
Use _DEFAULT_SOURCE, but retain _BSD_SOURCE for backwards
compatibility.

Thanks to Phil Sorber (PSUdaemon) for patch and investigation.

configure

@@ -599,7 +599,7 @@ elif test "$COMPILER" = "gcc" || test "$COMPILER" = "clang" || test "$COMPILER"
 		INSTALL_LIBS="install-lib"
 	fi
 
-	CFLAGS="-D_XOPEN_SOURCE=600 -D_BSD_SOURCE -std=gnu99 -pedantic -Wall -W -Wundef -Wendif-labels -Wshadow -Wpointer-arith -Wcast-align -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -Wnested-externs -Winline -Wdisabled-optimization -fstrict-aliasing -O2 -pipe -Wno-parentheses $CFLAGS"
+	CFLAGS="-D_XOPEN_SOURCE=600 -D_BSD_SOURCE -D_DEFAULT_SOURCE -std=gnu99 -pedantic -Wall -W -Wundef -Wendif-labels -Wshadow -Wpointer-arith -Wcast-align -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -Wnested-externs -Winline -Wdisabled-optimization -fstrict-aliasing -O2 -pipe -Wno-parentheses $CFLAGS"
 	PTHREAD_CFLAGS="-pthread"
 	if test "$COMPILER" = "mingw64"; then
 		ENVIRONMENT=64