Combine script more robust and can output to a specified file. Initial buck files added (work in progress).

This reverts commit 0df29a4e, reversing
changes made to 69c875a0.

Add --size-hint=# option

[fuzz] Add a DEBUGLOG(3) statement to print file

The script will combine decompressor sources into a single file. The example shows this in use.

Enable it by building with this command:

```
./fuzz.py build all --debug 3
```

Add --stream-size=# option

[legacy] Fix buffer overflow in v0.2 and v0.4 raw literals decompression

[fuzz] Improve fuzzer build script and docs

The CHECK_F macros differ slightly (but eventually do the same thing). Older GCC needs to fallback on the old-style pragma optimisation flags.

clarifications on field `Block_Size`

remove extraneous doubled ;s

fixed very minor inefficiency (nbSeq==127)

Fix the build on GCC 4.x after 812e8f2a

Extends the fix in PR#1722 to v0.2 and v0.4. These aren't built into
zstd by default, and v0.5 onward are not affected.

I only add the `srcSize > BLOCKSIZE` check to v0.4 because the comments
say that it must hold, but the equivalent comment isn't present in v0.2.

Credit to OSS-Fuzz.

* Remove the `make libFuzzer` target since it is broken and obsoleted
  by `CC=clang CXX=clang++ ./fuzz.py build all --enable-fuzzer`. The
  new `-fsanitize=fuzzer` is much better because it works with MSAN
  by default.
* Improve the `./fuzz.py gen` command by making the input type explicit
  when creating a new target.
* Update the `README` for `--enable-fuzzer`.

Fixes #1727.