[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/j6libc/is_leap.c

@@ -0,0 +1,14 @@
+/* _PDCLIB_is_leap( int )
+
+   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"
+
+int _PDCLIB_is_leap( int year_offset )
+{
+    /* year given as offset from 1900, matching tm.tm_year in <time.h> */
+    long long year = year_offset + 1900ll;
+    return ( ( year % 4 ) == 0 && ( ( year % 25 ) != 0 || ( year % 400 ) == 0 ) );
+}