vendor/libc: a bunch of additions

All these additions are to allow Dear ImGui to be compiled natively.

vendor/libc/include/alloca.h

@@ -0,0 +1,21 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#pragma once
+
+#include <stddef.h>
+
+void	*alloca(size_t);		/* built-in for gcc */
+
+#if defined(__GNUC__) && __GNUC__ >= 3
+/* built-in for gcc 3 */
+#undef	alloca
+#undef	__alloca
+#define	alloca(size)	__alloca(size)
+#define	__alloca(size)	__builtin_alloca(size)
+#endif
+
+#ifdef __cplusplus
+}
+#endif