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Detail:
 This changes all the C and assembler includes to be a canoncial Unix format.
 Also match include paths to previous commit for EditIntern/DrawIntern/VerIntern
 Finally, also include some minor type fixes (NULL vs 0)
Admin:
 May be some other paths elsewhere in the source I'm not immediately able to fix.  Will address any issues ASAP, since this is a huge change.

Version 5.54. Not tagged

          * isprint() now true for non-breaking spaces
<math.h>  * FP_INFINITY corrected to FP_INFINITE
          * fma(), fmal(), llrint(), llrintf(), llrintl(),
            llround(), llroundf(), llroundl(),
            remquo(), remquof(), remquol() added.
          * added inlining for remainder() and fmaf() and various
            float functions.
          * math_errhandling removed pending overhaul - it was misleading.
          * expm1() and log1p() now provide decent (< 1 ulp) accuracy,
            rather than only being marginally better than exp(x)-1 and
            log(1+x).
          * new version of copysign() with standard calling conventions.
          * some C99 functions changed to use infinity/NaNs and exceptions
            for error handling - see comments in header file about the
            changes in progress.
<stdio.h>  * output from %a now correctly rounded
<stdlib.h> * strtof() and strtold() added.
<string.h> * strerror() no longer clears _kernel_last_oserror().

Version 5.49. Tagged as 'RISC_OSLib-5_49'