feat: Set MACOSX_DEPLOYMENT_TARGET

[yutannihilation]

Problem

On macOS, R CMD check produces this linker warning:

ld: warning: object file (...unwind_protect_wrapper.o) was built for newer 'macOS'
version (26.2) than being linked (15.0)

unwind_protect_wrapper.o is compiled by the Rust cc crate (in savvy's build.rs).
Although we export CC and CFLAGS to cargo, the cc crate does not derive the
deployment target from these flags. It has its own lookup chain:
MACOSX_DEPLOYMENT_TARGET env var, then xcrun --show-sdk-version as fallback. Since
MACOSX_DEPLOYMENT_TARGET was not set, it fell back to the SDK version (26.2), while R's
clang defaulted to 15.0 (from its target triple arm64-apple-darwin24).

Fix

Detect the macOS deployment target in configure and export it via BEFORE_CARGO_BUILD
so the cc crate picks it up. Detection order:

1. MACOSX_DEPLOYMENT_TARGET env var (if already set)
2. -mmacosx-version-min= flag in R's CC/CFLAGS
3. __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ preprocessor macro from R's compiler

References

• clarabel-r: same fix
• tinyimg: -mmacosx-version-min detection