Support PATH variables with spaces

[ambv]

Without this change, make iOS and similar will fail when there's a space in PATH:

# Configure the build
cd build/iOS/libffi-3.4.2 && PATH=/Users/ambv/Dropbox/Python/Python-Apple-support/build/macOS/Python-3.11.0b1-macOS/_install/bin:/Users/ambv/.pyenv/shims:/Users/ambv/Library/Application Support/edgedb/bin:/Users/ambv/.dot_files/bin:/Users/ambv/.poetry/bin:/Users/ambv/.cargo/bin:/Users/ambv/.local/bin:/Library/TeX/texbin:/opt/homebrew/bin:/usr/local/opt/autoconf@2.69/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Library/Apple/usr/bin:/usr/local/MacGPG2/bin:/Library/TeX/texbin:/Users/ambv/opt/anaconda3/condabin python3.11 generate-darwin-source-and-headers.py --only-ios 2>&1 | tee -a ../libffi-iOS.config.log
/bin/sh: Support/edgedb/bin:/Users/ambv/.dot_files/bin:/Users/ambv/.poetry/bin:/Users/ambv/.cargo/bin:/Users/ambv/.local/bin:/Library/TeX/texbin:/opt/homebrew/bin:/usr/local/opt/autoconf@2.69/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Library/Apple/usr/bin:/usr/local/MacGPG2/bin:/Library/TeX/texbin:/Users/ambv/opt/anaconda3/condabin: No such file or directory
>>> Build libFFI for iphoneos.arm64
cd build/iOS/libffi-3.4.2/build_iphoneos-arm64 && make 2>&1 | tee -a ../../libffi-iphoneos.arm64.build.log
/bin/sh: line 0: cd: build/iOS/libffi-3.4.2/build_iphoneos-arm64: No such file or directory
make: *** [build/iOS/libffi-3.4.2/build_iphoneos-arm64/.libs/libffi.a] Error 1

Spaces are valid characters in PATH.

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