* Extract out common portion of `lib/Makefile` into `lib/libzstd.mk`.
Most relevantly, the way we find library files.
* Use `lib/libzstd.mk` in the other Makefiles instead of repeating the
same code.
* Add a test `tests/test-variants.sh` that checks that the builds of
`make -C programs allVariants` are correct, and run it in Actions.
* Adds support for ASM files in the CMake build.
The Meson build is not updated because it lists every file in zstd,
and supports ASM off the bat, so the Huffman ASM commit will just add
the ASM file to the list.
The Visual Studios build is not updated because I'm not adding ASM
support to Visual Studios yet.
pzstd requires C++11, which older C++ standard libraries like libstdc++
as used on OS X 10.8 and earlier don't support. The user might address
this by setting "CXXFLAGS=-stdlib=libc++". This flag must be used both
at compile time and at link time. Asking the user to also put the flag
in LDFLAGS is undesirable because then the flag would also be used when
linking C code, which would be inappropriate.
It can have other effects, such as pulling in extra libraries. Without it,
riscv build fails with undefined reference to `__atomic_compare_exchange_1'.
