macOS
I had this problem too (encountered through Macports compilers). Previous versions of Xcode would let you install command line tools through xcode/Preferences, but xcode5 doesn't give a command line tools option in the GUI, that so I assumed it was automatically included now. Try running this command:
xcode-select --install
If you see an error message that developer tools are already installed (and still header files can't be found), wipe out any existing one to do a fresh installation:
sudo rm -rf /Library/Developer/CommandLineTools
Ubuntu
(as per this answer)
sudo apt-get install libc6-dev
Alpine Linux
(as per this comment)
apk add libc-dev