Daniel - I don’t know if you have solved this problem already, but since I came across your post when looking for a solution to exactly the same issue, I thought I’d share what worked for me: putting the following in your Makevars file. Hope it solves your problem.
CFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
CCFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
CXXFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
CPPFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk