Runtime/math matches; better libc headers

Former-commit-id: bef7db1748

libc/assert.h

@@ -0,0 +1,20 @@
+#ifndef _ASSERT_H_
+#define _ASSERT_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if __STDC_VERSION__ >= 201112L
+// The C11 way
+#define static_assert(cond, msg) _Static_assert(cond, #msg)
+#else
+// The old, hacky way
+#define static_assert(cond, msg) typedef char static_assertion_##msg[(cond) ? 1 : -1]
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif