Move the meat of <features.h> into <sys/cdefs.h>.

This way it's a lot harder for us to screw up (since we should always
be including <sys/cdefs.h> anyway).

Bug: 14659579
Change-Id: I23070fff3296b0d1c683bb5e3a6e214146327d53

libc/include/sys/cdefs.h

@@ -355,9 +355,23 @@
 #define __RCSID(_s) /* nothing */
 #define __SCCSID(_s) /* nothing */
 
+/*
+ * _BSD_SOURCE and _GNU_SOURCE are expected to be defined by callers before
+ * any standard header file is included. In those header files we test
+ * against __USE_BSD and __USE_GNU. glibc does this in <features.h> but we
+ * do it in <sys/cdefs.h> instead because that's where our existing
+ * _POSIX_C_SOURCE tests were, and we're already confident that <sys/cdefs.h>
+ * is included everywhere it should be.
+ */
+#if defined(_BSD_SOURCE)
+# define __USE_BSD 1
+#endif
+
+#if defined(_GNU_SOURCE)
+# define __USE_GNU 1
+#endif
+
 /*-
- * The following definitions are an extension of the behavior originally
- * implemented in <sys/_posix.h>, but with a different level of granularity.
  * POSIX.1 requires that the macros we test be defined before any standard
  * header file is included.
  *

libc/include/sys/stat.h

@@ -29,7 +29,6 @@
 #ifndef _SYS_STAT_H_
 #define _SYS_STAT_H_
 
-#include <features.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>
 #include <sys/time.h>