tcc: Draft suppoprt for -MD/-MF options

In build systems, this is used to automatically collect target
dependencies, e.g.

    ---- 8< (hello.c) ----
    #include "hello.h"
    #include <stdio.h>

    int main()
    {
        printf("Hello World!\n");
        return 0;
    }

$ tcc -MD -c hello.c    # -> hello.o, hello.d
$ cat hello.d
hello.o : \
        hello.c \
        hello.h \
        /usr/include/stdio.h \
        /usr/include/features.h \
        /usr/include/bits/predefs.h \
        /usr/include/sys/cdefs.h \
        /usr/include/bits/wordsize.h \
        /usr/include/gnu/stubs.h \
        /usr/include/bits/wordsize.h \
        /usr/include/gnu/stubs-32.h \
        /home/kirr/local/tcc/lib/tcc/include/stddef.h \
        /usr/include/bits/types.h \
        /usr/include/bits/wordsize.h \
        /usr/include/bits/typesizes.h \
        /usr/include/libio.h \
        /usr/include/_G_config.h \
        /usr/include/wchar.h \
        /home/kirr/local/tcc/lib/tcc/include/stdarg.h \
        /usr/include/bits/stdio_lim.h \
        /usr/include/bits/sys_errlist.h \

NOTE: gcc supports -MD only for .c -> .o, but in tcc, we generate
dependencies for whatever action is being taken. E.g. for .c -> exe, the
result will be:

$ tcc -MD -o hello hello.c  # -> hello, hello.d
hello: \
        /usr/lib/crt1.o \
        /usr/lib/crti.o \
        hello.c \
        hello.h \
        /usr/include/stdio.h \
        /usr/include/features.h \
        /usr/include/bits/predefs.h \
        /usr/include/sys/cdefs.h \
        /usr/include/bits/wordsize.h \
        /usr/include/gnu/stubs.h \
        /usr/include/bits/wordsize.h \
        /usr/include/gnu/stubs-32.h \
        /home/kirr/local/tcc/lib/tcc/include/stddef.h \
        /usr/include/bits/types.h \
        /usr/include/bits/wordsize.h \
        /usr/include/bits/typesizes.h \
        /usr/include/libio.h \
        /usr/include/_G_config.h \
        /usr/include/wchar.h \
        /home/kirr/local/tcc/lib/tcc/include/stdarg.h \
        /usr/include/bits/stdio_lim.h \
        /usr/include/bits/sys_errlist.h \
        /usr/lib/libc.so \
        /lib/libc.so.6 \
        /usr/lib/ld-linux.so.2 \
        /lib/ld-linux.so.2 \
        /usr/lib/libc_nonshared.a \
        /lib/libc.so.6 \
        /usr/lib/libc_nonshared.a \
        /home/kirr/local/tcc/lib/tcc/libtcc1.a \
        /usr/lib/crtn.o \

So tcc dependency generator is a bit more clever than one used in gcc :)

Also, I've updated TODO and Changelog (in not-yet-released section).

v2:

(Taking inputs from grischka and me myself)

- put code to generate deps file into a function.
- used tcc_fileextension() instead of open-coding
- generate deps only when compilation/preprocessing was successful

v3:

- use pstrcpy instead of snprintf(buf, sizeof(buf), "%s", ...)

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