Fix 32-bit ARM build

[EyeOfPython]

Building this crate on arm-linux-gnueabihf is fine, HOWEVER, opening a rocksdb database using the built code fails:

On our CI, we get a "Value too large for defined data type" in our functional tests, see build output (you can login as guest if necessary).

After a bit of digging, it turns out this refers to a missing environment variable specifically for 32-bit systems that want to open files using 64-bit offsets.

Therefore, this PR fixes our build by adding those two env variables when CARGO_CFG_TARGET_POINTER_WIDTH is not 64.

[aleksuss]

I would propose to use matches! macro here rather than unwrap:

Suggested change

	if std::env::var("CARGO_CFG_TARGET_POINTER_WIDTH").unwrap() != "64" {
	if matches!(env::var("CARGO_CFG_TARGET_POINTER_WIDTH"), Ok(width) if width != "64") {

[EyeOfPython]

The other places also use unwrap, so I thunk this would be more consistent

[EyeOfPython]

I changed it to use env::var though, I think this way it looks the cleanest. CARGO_CFG_TARGET_POINTER_WIDTH is guaranteed to be defined in build scripts, so IMO it's better to have it panic and let us know something went terribly wrong.

[EyeOfPython]

We're currently blocking on this fix @aleksuss @stanislav-tkach, do you guys think it will be deployed soon?

[aleksuss]

I will merge it into the master, but a new release with this fix will appear after two or three months, I guess.

[EyeOfPython]

You can always release a minor version, like 0.20.2, or is there something preventing that?

[aleksuss]

You can always use a dependency directly from the master branch or even from a commit.

[Congyuwang]

This change seems to break windows x86 (i686) compile.

[Congyuwang]

running: "C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.35.32215\\bin\\HostX64\\x86\\cl.exe" "-nologo" "-MD" "-O2" "-Brepro" "-I" "rocksdb/include/" "-I" "rocksdb/" "-I" "rocksdb/third-party/gtest-1.8.1/fused-src/" "-I" "snappy/" "-I" "D:\\a\\RocksDict\\RocksDict\\target\\i686-pc-windows-msvc\\release\\build\\lz4-sys-2f2ef7caa05cef3e\\out\\include" "-I" "C:\\Users\\runneradmin\\.cargo\\registry\\src\\github.com-1ecc6299db9ec823\\zstd-sys-2.0.7+zstd.1.5.4\\zstd/lib" "-I" "D:\\a\\RocksDict\\RocksDict\\target\\i686-pc-windows-msvc\\release\\build\\libz-sys-a3360bed1d7b33fd\\out/include" "-I" "D:\\a\\RocksDict\\RocksDict\\target\\i686-pc-windows-msvc\\release\\build\\bzip2-sys-9491d39ce54a7111\\out\\include" "-I" "." "-W4" "-EHsc" "-std:c++17" "-DSNAPPY=1" "-DLZ4=1" "-DZSTD=1" "-DZLIB=1" "-DBZIP2=1" "-DNDEBUG=1" "-DDWIN32" "-DOS_WIN" "-D_MBCS" "-DWIN64" "-DNOMINMAX" "-DROCKSDB_WINDOWS_UTF8_FILENAMES" "-DROCKSDB_SUPPORT_THREAD_LOCAL" "-D_FILE_OFFSET_BITS=64" "-D_LARGEFILE64_SOURCE=1" "-FoD:\\a\\RocksDict\\RocksDict\\target\\i686-pc-windows-msvc\\release\\build\\librocksdb-sys-32d9bb9bc3d80072\\out\\rocksdb/cache/sharded_cache.o" "-c" "rocksdb/cache/sharded_cache.cc"
  sharded_cache.cc
  rocksdb/cache/lru_cache.h(482): warning C4324: 'rocksdb::lru_cache::LRUCacheShard': structure was padded due to alignment specifier
  D:\a\RocksDict\RocksDict\target\i686-pc-windows-msvc\release\build\libz-sys-a3360bed1d7b33fd\out\include\zconf.h(475): fatal error C1083: Cannot open include file: 'unistd.h': No such file or directory
  rocksdb/util/math.h(143): warning C4127: conditional expression is constant
  rocksdb/util/math.h(143): note: consider using 'if constexpr' statement instead
  rocksdb/cache/sharded_cache.cc(95): note: see reference to function template instantiation 'int rocksdb::BitsSetToOne<uint32_t>(T)' being compiled
          with
          [
              T=uint32_t
          ]
  rocksdb/util/math.h(153): warning C4127: conditional expression is constant
  rocksdb/util/math.h(153): note: consider using 'if constexpr' statement instead
  exit code: 2

It says 'unistd.h' not found for libz-sys.

Reverting this commit fix compiling on windows i686. Moreover, it also runs correctly on win32.

FYI: comparing these two Github Action Runs: https://github.com/Congyuwang/RocksDict/actions/runs/4476861228/jobs/7867755946 (reverted)
and before:
https://github.com/Congyuwang/RocksDict/actions/runs/4476651838/jobs/7867232892.