gem install unicorn 

出现错误如下： Temporarily enhancing PATH to include DevKit... Building native extensions. This could take a while... ERROR: Error installing unicorn: ERROR: Failed to build gem native extension.

D:/RailsInstaller/Ruby2.1.0/bin/ruby.exe extconf.rb checking for CLOCK_MONOTONIC in time.h... yes checking for clockid_t in time.h... yes checking for clock_gettime() in -lrt... no checking for t_open() in -lnsl... no checking for socket() in -lsocket... no checking for poll() in poll.h... no checking for getaddrinfo() in sys/types.h,sys/socket.h,netdb.h... no getaddrinfo required *** extconf.rb failed *** Could not create Makefile due to some reason, probably lack of necessary libraries and/or headers. Check the mkmf.log file for more details. You may need configuration options.

Provided configuration options: --with-opt-dir --without-opt-dir --with-opt-include --without-opt-include=${opt-dir}/include --with-opt-lib --without-opt-lib=${opt-dir}/lib --with-make-prog --without-make-prog --srcdir=. --curdir --ruby=D:/RailsInstaller/Ruby2.1.0/bin/ruby --with-rtlib --without-rtlib --with-nsllib --without-nsllib --with-socketlib --without-socketlib

extconf failed, exit code 1

Gem files will remain installed in D:/RailsInstaller/Ruby2.1.0/lib/ruby/gems/2.1.0/gems/kgio-2.10.0 for inspection. Results logged to D:/RailsInstaller/Ruby2.1.0/lib/ruby/gems/2.1.0/extensions/x86-mingw32/2.1.0/kgio-2.10.0/gem_make.out

请问是什么原因呢？