cc-wrapper, bintools-wrapper: support MACOSX_DEPLOYMENT_TARGET with roles

In a typical build environment the toolchain will use the value of the
MACOSX_DEPLOYMENT_TARGET environment variable to determine the version
of macOS to support. When cross compiling there are two distinct
toolchains, but they will look at this single environment variable. To
avoid contamination, we always set the equivalent command line flag
which effectively disables the toolchain's internal handling.

Prior to this change, the MACOSX_DEPLOYMENT_TARGET variable was
ignored, and the toolchains always used the Nix platform
definition (`darwinMinVersion`) unless overridden with command line
arguments.

This change restores support for MACOSX_DEPLOYMENT_TARGET, and adds
nix-specific MACOSX_DEPLOYMENT_TARGET_FOR_BUILD and
MACOSX_DEPLOYMENT_TARGET_FOR_TARGET for cross compilation.

pkgs/build-support/cc-wrapper/add-flags.sh

@@ -65,5 +65,13 @@ if [ -e @out@/nix-support/cc-cflags-before ]; then
     NIX_CFLAGS_COMPILE_BEFORE_@suffixSalt@="$(< @out@/nix-support/cc-cflags-before) $NIX_CFLAGS_COMPILE_BEFORE_@suffixSalt@"
 fi
 
+# Only add darwin min version flag if a default darwin min version is set,
+# which is a signal that we're targetting darwin.
+if [ "@darwinMinVersion@" ]; then
+    mangleVarSingle MACOSX_DEPLOYMENT_TARGET ${role_suffixes[@]+"${role_suffixes[@]}"}
+
+    NIX_CFLAGS_COMPILE_BEFORE_@suffixSalt@="-m@darwinPlatformForCC@-version-min=${MACOSX_DEPLOYMENT_TARGET_@suffixSalt@:-@darwinMinVersion@} $NIX_CFLAGS_COMPILE_BEFORE_@suffixSalt@"
+fi
+
 # That way forked processes will not extend these environment variables again.
 export NIX_CC_WRAPPER_FLAGS_SET_@suffixSalt@=1