Propagate Apple deployment target version from CMake to cc (#567)

* Propagate Apple deployment target version from CMake to `cc`

`rustc` always targets the earliest deployment platform version possible
for Rust code, but the `cc` crate defaults to the host's SDK version
which is typically very recent.

When the CMake uses a toolchain that has its' own minimal supported macOS
version (for example Qt), this leads to linker errors and possible crashes
when deploying to older yet supported systems.

cmake/Corrosion.cmake

@@ -703,6 +703,11 @@ function(_add_cargo_build out_cargo_build_out_dir)
         list(APPEND corrosion_cc_rs_flags "SDKROOT=${CMAKE_OSX_SYSROOT}")
     endif()
 
+    # Ensure that cc-rs targets same Apple platform version as the CMake build
+    if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_OSX_DEPLOYMENT_TARGET)
+        list(APPEND corrosion_cc_rs_flags "MACOSX_DEPLOYMENT_TARGET=${CMAKE_OSX_DEPLOYMENT_TARGET}")
+    endif()
+
     corrosion_add_target_local_rustflags("${target_name}" "$<$<BOOL:${corrosion_link_args}>:-Clink-args=${corrosion_link_args}>")
 
     # todo: this should probably also be guarded by if_not_host_build_condition.