These variables let us configure CMAKE_SYSROOT and
CMAKE_{C,CXX,ASM}_EXTERNAL_TOOLCHAIN from env variables
without needing an error-prone custom toolchain file.
Most users won't need BORING_BSSL_EXTERNAL_TOOLCHAIN, but some
packages (such as Homebrew package
messense/macos-cross-toolchains/x86_64-unknown-linux-gnu) don't
install the sysroot at the root of the GCC installation, so clang-12
cannot find crt1.o and crti.o.
Finally, we also set up CMAKE_CROSSCOMPILING and
CMAKE_{C,CXX,ASM}_COMPILER_TARGET to make cross compilation work
with compilers that have cross-compiling drivers (i.e. clang).
We can now cross build boring-sys from macOS to Linux with
fips feature turned on:
brew tap nox/misc
brew install llvm@12.0.0
export PATH="$(brew --prefix llvm@12.0.0)/bin:$PATH"
brew tap messense/macos-cross-toolchains
brew install x86_64-unknown-linux-gnu
export BORING_BSSL_FIPS_EXTERNAL_TOOLCHAIN="$(brew --prefix x86_64-unknown-linux-gnu)/toolchain"
export BORING_BSSL_FIPS_SYSROOT="$BORING_BSSL_FIPS_EXTERNAL_TOOLCHAIN/x86_unknown-linux-gnu/sysroot"
cargo build --target x86_64-unknown-linux-gnu -p boring-sys --features fips
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