script to link older macOS SDKs into newer Xcode so you can target older platforms properly

fixup-old-macos-sdks.sh

#!/bin/bash

# ---------------------------------------------------------------------------------------------------------
# place as many old macOS SDKs into the directory with this script e.g. if you have
#
#  my-macpro:SDKs natb$ ls -ahl
#  total 4254792
#  drwxr-xr-x  17 natb  admin   578B Nov 21  2017 .
#  drwxrwxr-x  24 root  admin   884B Aug 29 06:46 ..
#  drwxr-xr-x   5 natb  admin   170B Oct 17  2017 MacOSX10.10.sdk
#  drwxr-xr-x   5 natb  admin   170B Oct 17  2017 MacOSX10.11.sdk
#  drwxr-xr-x   5 natb  admin   170B Oct 17  2017 MacOSX10.12.sdk
#  drwxr-xr-x   5 natb  admin   170B Oct 17  2017 MacOSX10.13.sdk
#  drwxr-xr-x   7 natb  admin   238B Oct 17  2017 MacOSX10.6.sdk
#  drwxr-xr-x   7 natb  admin   238B Oct 17  2017 MacOSX10.7.sdk
#  drwxr-xr-x   5 natb  admin   170B Oct 17  2017 MacOSX10.8.sdk
#  drwxr-xr-x   5 natb  admin   170B Oct 17  2017 MacOSX10.9.sdk
#
# then run it to allow your current version of Xcode is to have access to SDKs back to 10.6
# tested with Xcode 8.x - 9.4.x. Works with Xcode 10, but Xcode 10 will not support 32-bit targets
# and complains indefinitely for 9.4.x and higher. I suggest 9.2.x as your stopping point if
# you plan to build universal/fat binaries that can run pre-10.14
# this same tactic works for iOS and tvOS SDKs.
# ---------------------------------------------------------------------------------------------------------

SDK_DIR="$(cd $(dirname $0) && echo $PWD)"

OSX_PLAT=`xcrun --sdk macosx --show-sdk-platform-path`
OSX_SDK=$OSX_PLAT/Developer/SDKs

pushd "$OSX_SDK"

# this links
sudo ln -sfn "$SDK_DIR"/* .

# this is a magic tweak for an ommission in the 10.9 SDK which can trip you up
if [ -e "$SDK_DIR/MacOSX10.10.sdk/usr/lib/crt1.10.6.o" -a ! -e "$SDK_DIR/MacOSX10.9.sdk/usr/lib/crt1.10.6.o" ]
then
  cp "$SDK_DIR/MacOSX10.10.sdk/usr/lib/crt1.10.6.o" "$SDK_DIR/MacOSX10.9.sdk/usr/lib/"
fi

popd

# this is the magic tweak to Info.plist to enable older SDKs that you have symlink'd in
sudo plutil -replace MinimumSDKVersion -string 10.6 "$OSX_PLAT"/Info.plist