xxhash: Update to 0.7.3. Fix installation of libs.

v0.7.3
- perf: improved speed for large inputs (~+20%)
- perf: improved latency for small inputs (~10%)
- perf: s390x Vectorial code, by @easyaspi314
- cli: improved support for Unicode filenames on Windows, thanks to @easyaspi314 and @t-mat
- api: `xxhash.h` can now be included in any order, with and without `XXH_STATIC_LINKING_ONLY` and `XXH_INLINE_ALL`
- build: xxHash's implementation transferred into `xxhash.h`. No more need to have `xxhash.c` in the `/include` directory for `XXH_INLINE_ALL` to work
- install: created pkg-config file, by @bket
- install: VCpkg installation instructions, by @LilyWangL
- doc: Highly improved code documentation, by @easyaspi314
- misc: New test tool in `/tests/collisions`: brute force collision tester for 64-bit hashes

v0.7.2
- Fixed collision ratio of `XXH128` for some specific input lengths, reported by @svpv
- Improved `VSX` and `NEON` variants, by @easyaspi314
- Improved performance of scalar code path (`XXH_VECTOR=0`), by @easyaspi314
- `xxhsum`: can generate 128-bit hashes with the `-H2` option (note: for experimental purposes only! `XXH128` is not yet frozen)
- `xxhsum`: option `-q` removes status notifications

v0.7.1
- Secret first: the algorithm computation can be altered by providing a "secret", which is any blob of bytes, of size >= `XXH3_SECRET_SIZE_MIN`.
- `seed` is still available, and acts as a secret generator
- updated `ARM NEON` variant by @easyaspi314
- Streaming implementation is available
- Improve compatibility and performance with Visual Studio, with help from @aras-p
- Better integration when using `XXH_INLINE_ALL`: do not pollute host namespace, use its own macros, such as `XXH_ASSERT()`, `XXH_ALIGN`, etc.
- 128-bit variant provides helper functions for comparison of hashes.
- Better `clang` generation of `rotl` instruction, thanks to @easyaspi314
- `XXH_REROLL` build macro to reduce binary size, by @easyaspi314
- Improved `cmake` script, by @Mezozoysky
- Full benchmark program provided in `/tests/bench`

(nia)