The previous 'patch the toolchain file' approach (234a798, 48d2bd4) did
write the SSE flags into the COMPILE_FLAGS list correctly in the baked
image, but the CMakeCache.txt from the libopus configure ended up
without them in CMAKE_C_FLAGS, so cmake's final compile commands
didn't see them either. Most plausible explanation: cmake-rs passes
`-DCMAKE_C_FLAGS=…` on the command line, and its assembly of that
string happens outside the toolchain's FORCE set path, so the
toolchain patch never propagated.
Switch to a different lever: cargo-xwin already ships a tiny
`override.cmake` loaded via CMAKE_USER_MAKE_RULES_OVERRIDE. That
file is the right place to manipulate the compile-command
`CMAKE_C_COMPILE_OBJECT` / `CMAKE_CXX_COMPILE_OBJECT` templates —
it runs after cmake has initialised its compile rules but before
any source is compiled. Append two string(REPLACE '<FLAGS>' '<FLAGS>
/clang:-msse4.1 /clang:-mssse3 /clang:-msse3 /clang:-msse2') lines
to that file so every C and C++ compile command generated by cmake
gets the SSE feature flags inline, no matter what the project's
CMAKE_C_FLAGS is set to.
This is the CMake equivalent of a compiler wrapper and works
regardless of how cmake-rs / cargo-xwin / libopus juggle their
respective flag variables.
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