Wasmer distro packaging notes

Where possible, do not directly invoke cargo, but use the supplied Makefile
- wasmer has several compiler backends and the Makefile autodetects whether to enable llvm and singlepass. Set ENABLE_{CRANELIFT,LLVM,SINGLEPASS}=1 to build the full set or fail trying
- Set WASMER_CAPI_USE_SYSTEM_LIBFFI=1 to force dynamic linking of libffi on the shared library
- make install respects DESTDIR, but prefix must be configured as e.g. WASMER_INSTALL_PREFIX=/usr make all
In case you must build/install directly with cargo, make sure to enable at least one compiler backend feature
- Beware that compiling with cargo build --workspace/--all --features ... will not enable features on the subcrates in the workspace and result in a headless wasmer binary that can not run wasm files directly.
If you split the package into several subpackages, beware that the create-exe command of wasmer requires libwasmer.a to be installed at $WASMER_INSTALL_PREFIX/lib/libwasmer.a. Suggestion for splitting:
- wasmer and wasmer-headless, containing the respective executables
  - wasmer-headless contains a subset of wasmer's functionality and should only be packaged when splitting - it must be built explicitly with make build-wasmer-headless-minimal install-wasmer-headless-minimal
- libwasmer, containing libwasmer.so*
- libwasmer-dev, containing the header files and a .pc file
- libwasmer-static, containing libwasmer.a

The wasmer distro packaging story is still in its infancy, so feedback is very welcome.

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