This helps gcc build since it assumes that string.h provides various
functions that POSIX mandates is in strings.h. This is OK with me since that
strings.h is a nasty hack anyways. Also, glibc does this.
The string returned is now const - POSIX did not allow modifying the string
in any case, conforming applications should not break. If _SORTIX_SOURCE is
defined strerror(3) automatically redirects to sortix_strerror(3),
otherwise the application will receive the traditional function.
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.
