'unistd.h' file not found for Xcode 7

fix-xcode7-command-line.md

After upgrading to Xcode7 and Command Line Tools 10.10, I can't compile standard programs anymore. The error is (from the cputhrottle package):

/tmp/cputhrottle% make
/usr/bin/g++ -c -g -Wall -I..///boost_1_43_0 cputhrottle.cc
In file included from cputhrottle.cc:5:
In file included from ./manip.h:4:
In file included from /usr/local/include/boost/shared_ptr.hpp:17:
In file included from /usr/local/include/boost/smart_ptr/shared_ptr.hpp:17:
In file included from /usr/local/include/boost/config.hpp:44:
In file included from /usr/local/include/boost/config/select_stdlib_config.hpp:18:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cstddef:36:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__config:23:10: fatal error:
      'unistd.h' file not found
#include <unistd.h>
         ^
1 error generated.
make: *** [cputhrottle.o] Error 1

Downloading and installed the Command Line Tools 10.10 for Xcode 6.4 fixed it, though.