Gem mysql2 installation error on ruby1.9.3 - ruby

Question

Just updated to macOS Mojave, reinstalled mysql via brew & downloaded latest xcode 9.2 . While triying to setup a development enviroment for an old rails project, bundle crashes on installation of mysql2 gem. In the beggining i thought it was because latest mysql2 version, is not compatible with ruby 1.9 so i tried:
gem install mysql2 -v '0.3.17'
and i received:
/Users/me/.rvm/rubies/ruby-1.9.3-p125/bin/ruby extconf.rb
checking for ruby/thread.h... * 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/me/.rvm/rubies/ruby-1.9.3-p125/bin/ruby
/Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:381:in try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:506:intry_cpp'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:931:in block in have_header'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:790:inblock in checking_for'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:284:in block (2 levels) in postpone'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:254:inopen'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:284:in block in postpone'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:254:inopen'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:280:in postpone'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:789:inchecking_for'
from /Users/me/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:930:in have_header'
from extconf.rb:9:in'
copy from /Users/me/.rvm/gems/ruby-1.9.3-p125#mailcatcher/gems/mysql2-0.2.18/ext/mysql2/mkmf.log:
"/usr/bin/gcc-4.2 -o conftest -I/Users/me/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1/x86_64-darwin10.8.0 -I/Users/me/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1/ruby/backward -I/Users/me/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1 -I. -I/Users/me/.rvm/usr/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -O3 -ggdb -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wshorten-64-to-32 -Wimplicit-function-declaration -fno-common -pipe conftest.c -L. -L/Users/me/.rvm/rubies/ruby-1.9.3-p125/lib -L/Users/me/.rvm/usr/lib -L. -L/usr/local/lib -lruby.1.9.1 -lpthread -ldl -lobjc "
checked program was:
/* begin /
1: #include "ruby.h"
2:
3: int main() {return 0;}
/ end */

Answer 1

It sounds like you're either missing the XCode command line tools:
xcode-select --install
Or mySQL packages:
If you don't already have homebrew:
$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
Then to install mysql:
brew install mysql
I can't remember which solved this issue for me in the past but I'm sure it was one of them.

Answer 2

Cross linking my answer to almost this exact same question. I think it might just be not finding the correct config path
sudo gem install mysql2 -- --with-mysql-config=/usr/local/bin/mysql_config
See the answer here
edit: formatting