* define targetos=Windows when --enable-tcc32-mingw, --enable-cygwin, ...
    * use TARGETOS insteed HOST_OS when selecting PROGS
    * use "$(tccdir)" insteed $(tccdir) on install (spaces in path)
    * install tcc.exe too
    * produce bcheck.o when cross-compiling too (lib/Makefile)
    * force bcheck.o linking by compiling inside tcc_set_output_type()
      a dummy program with local array. Otherwise bcheck.o may be not linked.
    * replace %xz format specifier with %p in bcheck (don't supported on
      Windows)
    * call a __bound_init when __bound_ptr_add, __bound_ptr_indir,
      __bound_new_region, __bound_delete_region called.
      This is because a __bound_init inside ".init" section is not called
      on Windows for unknown reason.
    * print on stderr a message when an illegal pointer is returned:
        there is no segmentation violation on Windows for a program
        compiled with "tcc -b"
    * remove "C:" subdir on clean if $HOST_OS = "Linux"
    * default CFLAGS="-Wall -g -O0" insteed CFLAGS="-Wall -g -O2"
      to speed up compilation and more precise debugging.