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.
This directory holds various "internals" of PDCLib: - definitions of helper functions not specified by the standard (hidden in the _PDCLIB_* namespace); - definitions of data objects, both internal (like _PDCLIB_digits) and specified by the standard (_PDCLIB_errno); - test drivers for functionality that does not have its own implementation file to put the test driver in (stdarg).