[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/locale/localeconv.c

@@ -0,0 +1,12 @@
+/* localeconv( void )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <locale.h>
+
+struct lconv * localeconv( void )
+{
+    return _PDCLIB_lc_numeric_monetary.lconv;
+}