Tweak TCPIP headers for more GCC friendliness
This is still a long way from being possible to compile with GCC. The main aim here is to facilitate preprocessing of TCPIPLibs clients so that we can run `cppcheck` on them. The problem being addressed is that the GCC preprocessor always looks preferentially for include files in the same directory as the one that features the `#include` statement. This means that when * `machine/limits.h` does `#include <limits.h>` * `machine/stdarg.h` does `#include <stdarg.h>` * `sys/signal.h` does `#include <signal.h>` * `sys/time.h` does `#include <time.h>` then, rather than including the top-level version of the header, as intended, GCC ends up in an infinite loop of inclusion. To deal with this with the minimum of side-effects, the recursive includes are now conditional on `__GNUC__` bein undefined - except `machine/stdarg.h` which adds nothing to `stdarg.h` (so it's recommended that where GCC compatibility is required, it is simply changed to `#include <stdarg.h>`). Where possible, instances of `#include <machine/limits.h>` within the TCPIPLibs headers have been prefixed with `#include <limits.h>`, and similarly for the others. The extra inclusion of the top-level header should be harmless for Norcroft compilation, but ensures that clients of those second-tier headers are less likely to need an additional `#include` of a top-level header in order to enable `cppcheck`.
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