I couldn't find a good way to spread `ip0` and `ip1` apart when we accelerate
due to incompressible inputs. (The methods I tried slowed things down quite a
bit.)

Since we aren't splaying ip0 and ip1 apart (which would be like `0_1_2_3_`, as
opposed to the `01__23__` we were actually doing), it's a big ambitious to
increment `step` by 2. Instead, let's increment it by 1, which has the benefit
sliiightly improving compression. Speed remains pretty much unchanged.

The position updates are rewritten from `ip[N] = ip[N-1] + step` to be
`ip[N] = ip[N-2] + step`. This lets us only deal with the asymmetric spacing
of gaps at setup and then we only have to keep a single `step` variable.

This seems to work quite well on GCC and Clang!

This avoids an additional addition, at the cost of an additional variable.

This replicates the behavior of @terrelln's `ZSTD_fast` implementation. That
is, it always looks at adjacent pairs of positions, and only applies the
acceleration every other position. This produces a more fine-grained
acceleration.

Update changelog for v1.5.1

Reject Irregular Dictionary Files

updated xxHash to latest v0.8.1

no idea why visual + clang-cl + appveyor don't like them,
I've not been able to reproduce the issue locally,
but these static assert are very unlikely to deliver a useful signal,
I can't imagine a situation where they will be wrong,
and if they are, then a ton of other things will be broken way before reaching that point.

Remove possible NULL pointer addition

I hadn't seen #2890, so I wrote my own version. I like this approach a little
better, since it does an explicit check for a regular file, rather than
passing a magic value.

Addresses #2874.

on top of implementations, which were already scoped out.

Refactor `ZSTDMT_isOverlapped()` to do NULL checks before computing the
end pointer.

Fixes #2906.

Fix oss-fuzz build

Fix Up #2659; Build libzstd.pc Whenever Building the Lib on Unix

Disable assembly when dataflow sanitizer is enabled.

This regressed in PR #2893, which accidentally removed the check for
dataflow sanitizer.

with minor modifications directly embedded in source :
- does not compile XXH3
- namespace emulation (ZSTD_ prefix)

Incidentally fix #2824

* Imply -q when stderr is not a tty

[lib] Fix libzstd.pc for lib-mt builds

Apply FORCE_MEMORY_ACCESS=1 to legacy

fixbug CLI's -D fails when the argument is not a regular file

See #2633, #2881.

[asm] Share portability macros and restrict ASM further

Avoid Using Deprecated Functions in Deprecated Code

Disable Multithreading in CMake Builds for Android

Move portability macros to `lib/common/portability_macros.h`. This file
only contains platform/feature detection (e.g. 0/1 macros). This file is
shared between C and ASM code, so it cannot include any C code.

Rename `HUF_` ASM macros to be `ZSTD_` prefixed, and move to the new
header.

Restrict `ZSTD_ASM_SUPPORTED` to `__GNUC__`, because we need the GAS
assembler.

Finally, only include the ASM code if we are actually going to use it.
This disables it on all Windows platforms, which should resolve the
problem brought up in Issue #2789.

[CI] Add cmake windows build

Improve zstd_opt build speed and size

Build on windows with cmake to ensure everything compiles.

Zstandard compiles and run on m68k cpus

Use the same trick as we did for zstd_lazy in PR #2828:
* Create one search function specialization for each (dictMode, mls).
* Select the search function pointer at the top of the match finder.

Additionally, we no longer inline `ZSTD_compressBlock_opt_generic` into
every function, since `dictMode` is no longer used as a template. Create
two specializations, for opt levels 0 and 2, and call one of the two
specializations.

Lastly, remove the hack that disabled inlining for zstd_opt for the
Linux Kernel, as we've gotten most of the benefit already.

Compilation time sees a ~4x reduction:

| Compiler | Flags                            | Dev Time (s) | PR Time (s) | Delta |
|----------|----------------------------------|--------------|-------------|-------|
| gcc      | -O3                              |         10.1 |         2.3 |  -77% |
| gcc      | -O3 -fsanitize=address,undefined |         61.1 |        10.2 |  -83% |
| clang    | -O3                              |          9.0 |         2.1 |  -76% |
| clang    | -O3 -fsanitize=address,undefined |         33.5 |         5.1 |  -84% |

Build size is reduced by 150KB - 200KB:

| Compiler | Dev libzstd.a Size (B) | PR libzstd.a Size (B) | Delta |
|----------|------------------------|-----------------------|-------|
| gcc      |                1327476 |               1177108 |  -11% |
| clang    |                1378324 |               1167780 |  -15% |

There is a <2% speed loss in all cases:

| Compiler | Level | Dev Speed (MB/s) | PR Speed (MB/s) | Delta  |
|----------|-------|------------------|-----------------|--------|
| gcc      |    16 |             4.78 |            4.72 | -1.25% |
| gcc      |    17 |             3.49 |            3.46 | -0.85% |
| gcc      |    18 |             2.92 |            2.86 | -2.04% |
| gcc      |    19 |             2.61 |            2.61 |  0.00% |
| clang    |    16 |             4.69 |            4.80 |  2.34% |
| clang    |    17 |             3.53 |            3.49 | -1.13% |
| clang    |    18 |             2.86 |            2.85 | -0.34% |
| clang    |    19 |             2.61 |            2.61 |  0.00% |

Fixes Issue #2862.