* Change the style of inclusion of the internal/ headers. Modern preprocessors

* Change the style of inclusion of the internal/ headers. Modern preprocessors
  all detect the header guards and know not to include anyhow. The old style may
  have actually been counterproductive
* Make the built library be standard independent (i.e. the library build is
  independent of _PDCLIB_C_VERSION/__STDC_VERSION__/_PDCLIB_CXX_VERSION
  /__cplusplus.
* <threads.h> from C11. A bit off of the track of C99 support (which is still a
  higher priority goal), but solid implementation of many C99 functions on many
  real platforms will need proper thread support. <stdio.h> support to come.
  This is a stub implementation.
* Move malloc/free/realloc etc to opt/malloc-solar, in preparation for support
  of other memory allocators. On the cards are at least ptmalloc3 (BSD) and
  dlmalloc (public domain) as options.
* New build system (See CoreMakefile.mk). Not ready yet; but its coming. This
  doesn't change the project dependencies - you still only need GNU make.

This is a large bundle of changes - most of them are as a result of implementing
PDClib into my system, so they're mostly "changes I was doing" that have got
wrapped up together. Hopefully, now that things are together, future changesets
should be better factored.

Special thanks go to Martin "Solar" Baute for the work he has put in on this
code over many years.