0.3.0: converge relibc to upstream 0.6.0 + Red Bear patches

tests/features.c

@@ -0,0 +1,50 @@
+// These tests are primarily to ensure the macros compile without 
+// causing any funny business.
+// TODO: does not compile with glibc
+
+#include <features.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#ifndef __GLIBC__
+__deprecated
+#endif
+static void legacy(void) {}
+
+#ifndef __GLIBC__
+__deprecatedNote("Sometimes deletes user's home (oops); use foobar")
+#endif
+static void legacy_notes(void) {}
+
+#ifndef __GLIBC__
+__nodiscard
+#endif
+static uint8_t the_answer(void) {
+    return 42;
+}
+
+// GCC bug
+/* #pragma GCC diagnostic push */
+/* #pragma GCC diagnostic ignored "-Wattributes" */
+/* __noreturn */
+/* static void foobar(void) { */
+    // The test suite isn't picking up noreturn in the headers for exit, abort
+    // Those functions (and this test) works fine in both Redox itself and Linux
+    // Using _Exit instead works for the tests in CI. Why? I dunno.
+/*     _Exit(0); */
+/* } */
+/* #pragma GCC diagnostic pop */
+
+int main(void) {
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+    legacy();
+    legacy_notes();
+    #pragma GCC diagnostic pop
+    const int answer = the_answer();
+    char buf[40] = {0};
+    sprintf(buf, "Hey, -Werror, I'm using answer: %d\n", answer);
+    /* foobar(); */
+    return 0;
+}