Auto Prompt for Installing Xcode CLI Tools

[rcdilorenzo]

I was running a shoestring file and I thought it might be nice if shoestring could figure out from the error message that Xcode CLI tools need to be installed and run xcode-select --install:

Fetching: nokogiri-1.6.6.2.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
    ERROR: Failed to build gem native extension.

        ~/.rbenv/versions/1.9.3-p374/bin/ruby extconf.rb
checking if the C compiler accepts ... yes
checking if the C compiler accepts -Wno-error=unused-command-line-argument-hard-error-in-future... no
Building nokogiri using packaged libraries.
-----
The file "/usr/include/iconv.h" is missing in your build environment,
which means you haven't installed Xcode Command Line Tools properly.

To install Command Line Tools, try running `xcode-select --install` on
terminal and follow the instructions.  If it fails, open Xcode.app,
select from the menu "Xcode" - "Open Developer Tool" - "More Developer
Tools" to open the developer site, download the installer for your OS
version and run it.
-----
*** 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=~/.rbenv/versions/1.9.3-p374/bin/ruby
    --help
    --clean
    --use-system-libraries
    --enable-static
    --disable-static
    --with-zlib-dir
    --without-zlib-dir
    --with-zlib-include
    --without-zlib-include=${zlib-dir}/include
    --with-zlib-lib
    --without-zlib-lib=${zlib-dir}/lib
    --enable-cross-build
    --disable-cross-build


Gem files will remain installed in ~/.rbenv/versions/1.9.3-p374/lib/ruby/gems/1.9.1/gems/nokogiri-1.6.6.2 for inspection.
Results logged to ~/.rbenv/versions/1.9.3-p374/lib/ruby/gems/1.9.1/gems/nokogiri-1.6.6.2/ext/nokogiri/gem_make.out