bintools-wrapper, cc-wrapper: parameterize darwin min version variable

These variables are the ones that the standard toolchain uses, so we
should use those and not always use MACOSX_DEPLOYMENT_TARGET.

See https://github.com/tpoechtrager/cctools-port/blob/236a426c1205a3bfcf0dbb2e2faf2296f0a100e5/cctools/ld64/src/ld/PlatformSupport.cpp#L54-L55

pkgs/build-support/bintools-wrapper/add-darwin-ldflags-before.sh

@@ -21,7 +21,7 @@ havePlatformVersionFlag=
 haveDarwinSDKVersion=
 haveDarwinPlatformVersion=
 
-mangleVarSingle MACOSX_DEPLOYMENT_TARGET ${role_suffixes[@]+"${role_suffixes[@]}"}
+mangleVarSingle @darwinMinVersionVariable@ ${role_suffixes[@]+"${role_suffixes[@]}"}
 
 n=0
 nParams=${#params[@]}
@@ -61,12 +61,12 @@ done
 if [ ! "$havePlatformVersionFlag" ]; then
     if [ ! "$haveDarwinSDKVersion" ] && [ ! "$haveDarwinPlatformVersion" ]; then
         # Nothing provided. Use the modern "-platform_version" to set both.
-        extraBefore+=(-platform_version @darwinPlatform@ "${MACOSX_DEPLOYMENT_TARGET_@suffixSalt@:-@darwinMinVersion@}" @darwinSdkVersion@)
+        extraBefore+=(-platform_version @darwinPlatform@ "${@darwinMinVersionVariable@_@suffixSalt@:-@darwinMinVersion@}" @darwinSdkVersion@)
     elif [ ! "$haveDarwinSDKVersion" ]; then
         # Add missing sdk version
         extraBefore+=(-sdk_version @darwinSdkVersion@)
     elif [ ! "$haveDarwinPlatformVersion" ]; then
         # Add missing platform version
-        extraBefore+=(-@darwinPlatform@_version_min "${MACOSX_DEPLOYMENT_TARGET_@suffixSalt@:-@darwinMinVersion@}")
+        extraBefore+=(-@darwinPlatform@_version_min "${@darwinMinVersionVariable@_@suffixSalt@:-@darwinMinVersion@}")
     fi
 fi

pkgs/build-support/bintools-wrapper/default.nix

@@ -327,11 +327,14 @@ stdenv.mkDerivation {
     ###
     + optionalString stdenv.targetPlatform.isDarwin (
       let
-        inherit (stdenv.targetPlatform) darwinMinVersion darwinPlatform darwinSdkVersion;
+        inherit (stdenv.targetPlatform)
+          darwinPlatform darwinSdkVersion
+          darwinMinVersion darwinMinVersionVariable;
       in ''
         export darwinPlatform=${darwinPlatform}
         export darwinMinVersion=${darwinMinVersion}
         export darwinSdkVersion=${darwinSdkVersion}
+        export darwinMinVersionVariable=${darwinMinVersionVariable}
         substituteAll ${./add-darwin-ldflags-before.sh} $out/nix-support/add-local-ldflags-before.sh
       ''
     )