I recently upgraded my Xcode command line tools and now I get errors when compiling stuff that compiled before the upgrade.
tools/px.c:7:10: fatal error: 'unistd.h' file not found
#include <unistd.h>
^~~~~~~~~~
Before this upgrade, whenever I installed a new release of Xcode, I had to install the command line tools and additionally
/Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg
After the latest upgrade, that file and the directory containing it does not exist any more.