Fix warnings on Linux

[finestructure]

I've only been able to test this with Swift 5.5 on macOS and Linux: both build and pass tests without warnings.

I still have the Makefile around from when I started looking into this to test more platforms but I've since moved to an M1 and don't have hardware within easy reach to actually run the targets 😆

Not sure what automated testing there is for the repo but I can make an effort to run the full matrix if it helps. Let me know!

clean:
    @# just delete specific directories instead of .builds so we don't
    @# have to re-fetch dependencies
    @rm -f .build/debug
    @rm -rf .build/x86_64-apple-macosx/
    @rm -rf .build/x86_64-unknown-linux/
    @rm -rf .build/x86_64-unknown-linux-gnu/

macos-spm-4.2: clean
    @echo
    @echo === Building $@ ===
    env DEVELOPER_DIR=/Applications/Xcode_10.1 xcrun swift build

macos-spm-5.0: clean
    @echo
    @echo === Building $@ ===
    env DEVELOPER_DIR=/Applications/Xcode_10.3.app xcrun swift build

macos-spm-5.1: clean
    @echo
    @echo === Building $@ ===
    env DEVELOPER_DIR=/Applications/Xcode_11.3.1.app xcrun swift build

macos-spm-5.2: clean
    @echo
    @echo === Building $@ ===
    env DEVELOPER_DIR=/Applications/Xcode_11.6.app xcrun swift build

macos-spm-5.3: clean
    @echo
    @echo === Building $@ ===
    env DEVELOPER_DIR=/Applications/Xcode_12_beta_5.app xcrun swift build

linux-4.2: clean
    @echo
    @echo === Building $@ ===
    docker run --rm -v "$(PWD)":/host -w /host swift:4.2.4 swift build

linux-5.0: clean
    @echo
    @echo === Building $@ ===
    docker run --rm -v "$(PWD)":/host -w /host swift:5.0.3 swift build

linux-5.1: clean
    @echo
    @echo === Building $@ ===
    docker run --rm -v "$(PWD)":/host -w /host swift:5.1.5 swift build --enable-test-discovery

linux-5.2: clean
    @echo
    @echo === Building $@ ===
    docker run --rm -v "$(PWD)":/host -w /host swift:5.2.4 swift build --enable-test-discovery

linux-5.3: clean
    @echo
    @echo === Building $@ ===
    docker run --rm -v "$(PWD)":/host -w /host "swiftlang/swift:nightly-5.3@sha256:99cd46394189f06ee7f7221d9873f273e5d0d4efe89824d201b0d9838aec4f50" swift build --enable-test-discovery

#   macos-spm-4.2 \
    macos-spm-5.0 
all: \
    macos-spm-5.1 \
    macos-spm-5.2 \
    macos-spm-5.3 \
    linux-4.2 \
    linux-5.0 \
    linux-5.1 \
    linux-5.2 \
    linux-5.3