Add minimal basic features.h resembling musl lib's own

include/features.h

+/*
+ * MIT License
+ * Copyright (c) 2020 Rich Felker musl-libc
+ */
+
+#ifndef _FEATURES_H
+#define _FEATURES_H
+
+#if __STDC_VERSION__ >= 199901L
+#define __restrict restrict
+#elif !defined(__GNUC__)
+#define __restrict
+#endif
+
+#if __STDC_VERSION__ >= 199901L || defined(__cplusplus)
+#define __inline inline
+#elif !defined(__GNUC__)
+#define __inline
+#endif
+
+#if __STDC_VERSION__ >= 201112L
+#elif defined(__GNUC__)
+#define _Noreturn __attribute__((__noreturn__))
+#else
+#define _Noreturn
+#endif
+
+#endif