Building native extensions.  This could take a while...
ERROR:  Error installing iStats:
ERROR: Failed to build gem native extension.

current directory: /Library/Ruby/Gems/2.3.0/gems/iStats-1.6.1/ext/osx_stats
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20190117-44665-taivle.rb extconf.rb
creating Makefile

current directory: /Library/Ruby/Gems/2.3.0/gems/iStats-1.6.1/ext/osx_stats
make "DESTDIR=" clean

current directory: /Library/Ruby/Gems/2.3.0/gems/iStats-1.6.1/ext/osx_stats
make "DESTDIR="
make: *** No rule to make target `/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0/universal-darwin16/ruby/config.h', needed by `smc.o'.  Stop.

make failed, exit code 2

Gem files will remain installed in /Library/Ruby/Gems/2.3.0/gems/iStats-1.6.1 for inspection.
Results logged to /Library/Ruby/Gems/2.3.0/extensions/universal-darwin-16/2.3.0/iStats-1.6.1/gem_make.out