macOS 26 moved libc++ headers inside the SDK, breaking Apple clang 17's
default header search. The boring-sys cmake step fails with
'fatal error: memory file not found' when building --features stealth.
Adding CXXFLAGS and SDKROOT to .cargo/config.toml [env] propagates them
to all build scripts (Cargo's documented mechanism), so cmake picks up the
SDK's C++ include path without any manual env-var ceremony.
force=false preserves CI overrides and Xcode users who already have
SDKROOT set. Non-macOS platforms ignore SDKROOT; clang silently skips
-isystem paths that do not exist on disk.
Fixes#136
