Set AR_<triple> for cc-rs

- Set the `AR_<triple>` variable for cc-rs.
- Simplify condition for CC_<triple> and CXX_<triple>.
  The target is always defined in corrosion,
  so there is no need to check if it set.

Closes #454

cmake/Corrosion.cmake

@@ -774,15 +774,19 @@ function(_add_cargo_build out_cargo_build_out_dir)
 
     set(corrosion_cc_rs_flags)
 
-    if(CMAKE_C_COMPILER AND _CORROSION_RUST_CARGO_TARGET_UNDERSCORE)
+    if(CMAKE_C_COMPILER)
         # This variable is read by cc-rs (often used in build scripts) to determine the c-compiler.
         # It can still be overridden if the user sets the non underscore variant via the environment variables
         # on the target.
         list(APPEND corrosion_cc_rs_flags "CC_${_CORROSION_RUST_CARGO_TARGET_UNDERSCORE}=${CMAKE_C_COMPILER}")
     endif()
-    if(CMAKE_CXX_COMPILER AND _CORROSION_RUST_CARGO_TARGET_UNDERSCORE)
+    if(CMAKE_CXX_COMPILER)
         list(APPEND corrosion_cc_rs_flags "CXX_${_CORROSION_RUST_CARGO_TARGET_UNDERSCORE}=${CMAKE_CXX_COMPILER}")
     endif()
+    if(CMAKE_AR)
+        list(APPEND corrosion_cc_rs_flags "AR_${_CORROSION_RUST_CARGO_TARGET_UNDERSCORE}=${CMAKE_AR}")
+    endif()
+
     # Since we instruct cc-rs to use the compiler found by CMake, it is likely one that requires also
     # specifying the target sysroot to use. CMake's generator makes sure to pass --sysroot with
     # CMAKE_OSX_SYSROOT. Fortunately the compilers Apple ships also respect the SDKROOT environment