Drop project-wide -D_XOPEN_SOURCE=700 after 63a20cf

- usleep was removed in POSIX.1-2008 (X/Open Issue 7)
- DT_DIR and flock are BSD extensions but BSDs don't use _DEFAULT_SOURCE
- _DEFAULT_SOURCE in GNU exposes POSIX.1-2008 + BSD + SysV mix

Makefile

@@ -14,7 +14,7 @@ CFLAGS:=-DVERSION=\"v$(VERSION)\ \($(COMMIT)\)\" \
 	-Wno-unused \
 	-std=c11 \
 	-D_DEFAULT_SOURCE \
-	-D_XOPEN_SOURCE=700 $(CFLAGS)
+	$(CFLAGS)
 
 platform=$(shell uname -s)