message: feat(makefile) Use target-specific variable for `RUSTFLAGS`.
  Context
  By defining a target-specific variable named `RUSTFLAGS`, the user can
  edit it, or set a pre-defined value for it.
  Moreover, this patch removes the global `RUSTFLAGS` variable only used
  by `link-packages`, and makes it target-specific.
diff: @@ -354,8 +354,9 @@ build-wasmer-debug:
  # incremental = false
  # codegen-units = 1
  # rpath = false
  +build-wasmer-headless-minimal: RUSTFLAGS += "-C panic=abort"
  build-wasmer-headless-minimal:
  - RUSTFLAGS="-C panic=abort" xargo build --target $(HOST_TARGET) --release --manifest-path=lib/cli/Cargo.toml --no-default-features --features headless-minimal --bin wasmer-headless
  + RUSTFLAGS=${RUSTFLAGS} xargo build --target $(HOST_TARGET) --release --manifest-path=lib/cli/Cargo.toml --no-default-features --features headless-minimal --bin wasmer-headless
  ifeq ($(IS_DARWIN), 1)
  strip -u target/$(HOST_TARGET)/release/wasmer-headless
  else
  @@ -708,7 +709,7 @@ install-wasmer-headless-minimal:
  update-testsuite:
  git subtree pull --prefix tests/wast/spec https://github.com/WebAssembly/testsuite.git master --squash
  -RUSTFLAGS := "-D dead-code -D nonstandard-style -D unused-imports -D unused-mut -D unused-variables -D unused-unsafe -D unreachable-patterns -D bad-style -D improper-ctypes -D unused-allocation -D unused-comparisons -D while-true -D unconditional-recursion -D bare-trait-objects" # TODO: add `-D missing-docs` # TODO: add `-D function_item_references` (not available on Rust 1.47, try when upgrading)
  +link-packages: RUSTFLAGS += "-D dead-code -D nonstandard-style -D unused-imports -D unused-mut -D unused-variables -D unused-unsafe -D unreachable-patterns -D bad-style -D improper-ctypes -D unused-allocation -D unused-comparisons -D while-true -D unconditional-recursion -D bare-trait-objects" # TODO: add `-D missing-docs` # TODO: add `-D function_item_references` (not available on Rust 1.47, try when upgrading)
  lint-packages:
  RUSTFLAGS=${RUSTFLAGS} cargo clippy -p wasmer
  RUSTFLAGS=${RUSTFLAGS} cargo clippy -p wasmer-c-api
  