Preprocessor uses libnix header files when no CRT library is specified

[cahirwpz]

Without -noixemul or -mcrt=* preprocessor should fail to find stdlib.h, because the header file is a part of standard C library (either nix or clib2). Following example should fail:

$ cat 0.c
#include <stdlib.h>
#include <stddef.h>
$ PATH=/opt/m68k-amigaos/bin:$PATH m68k-amigaos-gcc -c 0.c

Conclusion: $PREFIX/m68k-amigaos/sys-include should be dropped!