Ilya Leoshkevich

I need to rebase this, this part has already been committed as https://github.com/zlib-ng/zlib-ng/commit/9be98893aaf69892811d617122e28ea2dd8e17ba (apparently more is needed to solve the problem in a generic way, I was just concerned with...

Benchmarking has shown that inline history doesn't help much, so we'll most likely have to go with this solution. I'll let it sit in draft status until MSan and concurrency...

Gentle ping.

* Fixed a bug when `Z_SYNC_FLUSH` usage led to incomplete EOBS write. * Fixed a "goto fail" bug in `dfltcc_deflate_get_dictionary()`. * Replaced `getenv()` with `secure_getenv()`. * Added `sys/sdt.h` feature test.

* Fixed incorrect usage of STFLE instruction (reported against gzip [here](https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg00113.html), fixed in gzip [here](http://lists.gnu.org/archive/html/bug-gzip/2019-06/msg00000.html), corresponding zlib-ng pull request [here](https://github.com/zlib-ng/zlib-ng/pull/357)). * Simplified hooks by removing `#undef` statements.

* Fixed 31-bit build: * Added machine mode hint for STFLE. * Adjusted offset calculations. * Fixed `sys/sdt.h` feature test. * Added an entry to `contrib/README.contrib`.

* Added support for switching between software and hardware compression. * Added `--dfltcc` configure flag (the old way of building it still works).

* Fix missing EOBS in raw streams. * Parse environment variables and facility bits only once.

* Fix a number of issues related to switching compression levels. * Fix building with clang. * Fix sys/sdt.h detection.

* Make `inflateSyncPoint()` gracefully fail instead of returning an incorrect result.