Installing Wit gem causes obscure Ruby error - ruby-on-rails

Question

I created a brand new Rails app and added gem wit to the Gemfile. Now, when I run bundle install, I get this error. In Googling to find a solution, I came across four different possibilities but none of them apply to my dev environment (OSX 10.9.5, Brew 0.9.5). Has anyone solved this issue?
Links:
The compiler failed to generate an executable file. (RuntimeError)
https://github.com/CocoaPods/CocoaPods/issues/1727
https://github.com/copiousfreetime/hitimes/issues/26
https://teamtreehouse.com/forum/rails-gem-wont-install-properly
...
Using turbolinks 2.5.2
Using uglifier 2.5.3
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
/Users/user/.rvm/rubies/ruby-2.1-head/bin/ruby -r ./siteconf20141204-96223-d2ceuv.rb extconf.rb
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 12.7M 100 12.7M 0 0 741k 0 0:00:17 0:00:17 --:--:-- 723k
checking for main() in -lsox... *** 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
--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=/Users/user/.rvm/rubies/ruby-2.1-head/bin/ruby
--with-wit-dir
--without-wit-dir
--with-wit-include
--without-wit-include=${wit-dir}/include
--with-wit-lib
--without-wit-lib=${wit-dir}/lib
--with-soxlib
--without-soxlib
/Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:541:in `try_link0'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:556:in `try_link'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:742:in `try_func'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:973:in `block in have_library'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:918:in `block in checking_for'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:351:in `block (2 levels) in postpone'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:321:in `open'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:351:in `block in postpone'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:321:in `open'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:347:in `postpone'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:917:in `checking_for'
from /Users/user/.rvm/rubies/ruby-2.1-head/lib/ruby/2.1.0/mkmf.rb:968:in `have_library'
from extconf.rb:33:in `<main>'
extconf failed, exit code 1
Gem files will remain installed in /Users/user/.rvm/gems/ruby-2.1-head/gems/wit-1.0.5 for inspection.
Results logged to /Users/user/.rvm/gems/ruby-2.1-head/extensions/x86_64-darwin-13/2.1.0/wit-1.0.5/gem_make.out
An error occurred while installing wit (1.0.5), and Bundler cannot continue.
Make sure that `gem install wit -v '1.0.5'` succeeds before bundling.
Here is the output for mkmf.log
$ cat /Users/user/.rvm/gems/ruby-2.1-head/extensions/x86_64-darwin-13/2.1.0/wit-1.0.5/mkmf.log
"/usr/bin/clang -o conftest -I/Users/user/.rvm/rubies/ruby-2.1-head/include/ruby-2.1.0/x86_64-darwin13.0 -I/Users/user/.rvm/rubies/ruby-2.1-head/include/ruby-2.1.0/ruby/backward -I/Users/user/.rvm/rubies/ruby-2.1-head/include/ruby-2.1.0 -I. -I/Users/user/.rvm/gems/ruby-2.1-head/gems/wit-1.0.5/ext/wit/libwit/include -I/Users/user/.rvm/rubies/ruby-2.1-head/include -I/usr/local/opt/libyaml/include -I/usr/local/opt/readline/include -I/usr/local/opt/libksba/include -I/usr/local/opt/openssl/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wshorten-64-to-32 -Wimplicit-function-declaration -Wdivision-by-zero -Wextra-tokens -fno-common -pipe conftest.c -L. -L/Users/user/.rvm/rubies/ruby-2.1-head/lib -L/Users/user/.rvm/gems/ruby-2.1-head/gems/wit-1.0.5/ext/wit/libwit/lib -L/Users/user/.rvm/rubies/ruby-2.1-head/lib -L/usr/local/opt/libyaml/lib -L/usr/local/opt/readline/lib -L/usr/local/opt/libksba/lib -L/usr/local/opt/openssl/lib -L. -fstack-protector -L/usr/local/lib -L/usr/local/opt/libyaml/lib -L/usr/local/opt/readline/lib -L/usr/local/opt/libksba/lib -L/usr/local/opt/openssl/lib -lwit -lsox -lcurl -lruby.2.1.0 -lpthread -lgmp -ldl -lobjc "
ld: library not found for -lsox
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

I got this problem solved by reinstalling X-Code's Command Line tools: xcode-install --select