library not found for -lgmp error when installing Rails on El Capitan ( Mac OS 10.11.1 (15B42))

Question

When installing Rails on El Capitan (Mac OS X 10.11.1) on Ruby v2.2.2, I got the following error:

ERROR:  Error installing nokogiri:
    ERROR: Failed to build gem native extension.

    /Users/jon/.rvm/rubies/ruby-2.2.2/bin/ruby -r ./siteconf20151117-26799-ux15fd.rb extconf.rb --use-system-libraries
checking if the C compiler accepts ... *** 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.

And the mkmf.log file had:

$ cat /Users/jon/.rvm/rubies/ruby-2.2.2/lib/ruby/gems/2.2.0/extensions/x86_64-darwin-14/2.2.0/nokogiri-1.6.6.3/mkmf.log 
"gcc -o conftest -I/Users/jon/.rvm/rubies/ruby-2.2.2/include/ruby-2.2.0/x86_64-darwin14 ...
-lruby.2.2.0  -lpthread -lgmp -ldl -lobjc  "
ld: library not found for -lgmp
clang: error: linker command failed with exit code 1 (use -v to see invocation)
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: int main(int argc, char **argv)
4: {
5:   return 0;
6: }
/* end */

Answer 1

FIX

xcode-select --install

And re-running the gem install rails worked.