[libc] Bring libc in-tree

Moving libc from its separate repo into this one, minor resulting build
fixes, and a hacky way to add -I for libc headers in builds.

src/libraries/libc/include/limits.h

@@ -0,0 +1,36 @@
+#pragma once
+/* Sizes of integer types <limits.h>
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include "j6libc/int.h"
+
+/* TODO: Defined to 1 as multibyte characters are not supported yet. */
+#define MB_LEN_MAX 1
+
+#define SCHAR_MAX  __SCHAR_MAX__
+#define SCHAR_MIN  ((-SCHAR_MAX) - 1)
+#define UCHAR_MAX  (((unsigned char)SCHAR_MAX << 1) | 1)
+
+#define CHAR_BIT   __CHAR_BIT__
+#define CHAR_MAX   SCHAR_MAX
+#define CHAR_MIN   SCHAR_MIN
+
+#define SHRT_MAX   __SHRT_MAX__
+#define SHRT_MIN   ((-SHRT_MAX) - 1)
+#define USHRT_MAX  (((unsigned short)SHRT_MAX << 1) | 1)
+
+#define INT_MAX    __INT_MAX__
+#define INT_MIN    ((-INT_MAX) - 1)
+#define UINT_MAX   (((unsigned int)INT_MAX << 1) | 1)
+
+#define LONG_MAX   __LONG_MAX__
+#define LONG_MIN   ((-LONG_MAX) - 1)
+#define ULONG_MAX  (((unsigned long)LONG_MAX << 1) | 1)
+
+#define LLONG_MAX  __LONG_LONG_MAX__
+#define LLONG_MIN  ((-LLONG_MAX) - 1)
+#define ULLONG_MAX (((unsigned long long)LLONG_MAX << 1) | 1)
+