Change the story about POSIX headers in C compilation.

Until now, all C code in the RTS and library cbits has by default been
compiled with settings for POSIXness enabled, that is:
   #define _POSIX_SOURCE   1
   #define _POSIX_C_SOURCE 199309L
   #define _ISOC9X_SOURCE
If you wanted to negate this, you'd have to define NON_POSIX_SOURCE
before including headers.

This scheme has some bad effects:

* It means that ccall-unfoldings exported via interfaces from a
  module compiled with -DNON_POSIX_SOURCE may not compile when
  imported into a module which does not -DNON_POSIX_SOURCE.

* It overlaps with the feature tests we do with autoconf.

* It seems to have caused borkage in the Solaris builds for some
  considerable period of time.

The New Way is:

* The default changes to not-being-in-Posix mode.

* If you want to force a C file into Posix mode, #include as
  the **first** include the new file ghc/includes/PosixSource.h.
  Most of the RTS C sources have this include now.

* NON_POSIX_SOURCE is almost totally expunged.  Unfortunately
  we have to retain some vestiges of it in ghc/compiler so that
  modules compiled via C on Solaris using older compilers don't
  break.