C and C++ files are now kept together and so are the mxmpp declarations.
Header files are now stored in include/ and mxmpp'd into preproc/.
All other code now -I ../libmaxsi/preproc.
And other stuff to make this happen, including refactoring Makefile.
<features.h> declares _SORTIX_SOURCE if no conflicting macros are
declared, such as _GNU_SOURCE.
Fixed g++ automatically declaring _GNU_SOURCE, but Sortix isn't GNU.
Replaced SORTIX_UNIMPLEMENTED macro with __SORTIX_SHOW_UNIMPLEMENTED.
