This SO answer helped me.
Show the package contents of your project file (MyProject.xcodeproj) by two finger clicking on it in finder, then open the 'project.pbxproj' file in a text editor.
Look for the section XCConfigurationList. It starts with /* Begin XCConfigurationList section */
. You will find all your targets and their respective builConfigurations. Now do a find (command - f) on the long hex ID to find the other occurance in the project.pbxproj where you should also find your buildSettings. Copy and paste everything between the buildSettings brackets into your xcconfig file. You will then need to massage some of the variables, specifically the lists of search paths.
Alternatively, you can use xcodebuild from the command line:
First to list your schemes:
xcodebuild -list
Then export your desired scheme's settings:
xcodebuild -scheme "schemeName" -showBuildSettings >> mynew.xcconfig
You will then need to delete the first line or comment it out
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