When setting the iOS Deployment Target to be lower than 6.0, Xcode's gcc will fail with the error
ld: library not found for -lcrt1.3.1.o
It is possible to copy this file and others from an older Xcode version to get it working, but that's not always an option.
This project provides a minimal Makefile to build and install the missing files in the current Xcode installation. It uses Csu-85 for the build.
sudo make install
Csu-85 from https://opensource.apple.com/tarballs/Csu/