gem install mysql2 missing mysql.h on OS X

Ok. The problem surfaced after I needed Ben to help on a site I was building.
He didn't have ruby or mysql installed on his laptop.
The site was built using Ruby-2.0.0-p195 and Rails-4.0.0-rc1.
That, in itself, isn't the issue.

We went and installed the x86_64 version of MySQL 5.6 on his machine.
This sets up in /usr/local/mysql.
Got a bit annoyed as the install doesn't have any StartupItems so start/stop is manual.
Ce la vie.

I now installed Ruby-2.0.0-p195 with no dramas.

The site code was loaded up into a folder and "bundle install" was run.
That's when the fun started.
The mysql2 gem wouldn't compile.
You get something like this:

This could take a while...
ERROR:  Error installing mysql2:
 ERROR: Failed to build gem native extension.

    /usr/local/rvm/rubies/ruby-2.0.0-p195/bin/ruby extconf.rb
checking for rb_thread_blocking_region()... yes
checking for rb_wait_for_single_fd()... yes
checking for mysql.h... no
checking for mysql/mysql.h... no
-----
mysql.h is missing.  please check your installation of mysql and try again.
-----
*** 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=/usr/local/rvm/rubies/ruby-2.0.0-p195/bin/ruby
 --with-mysql-config
 --without-mysql-config


Gem files will remain installed in /usr/local/rvm/gems/ruby-2.0.0-p195/gems/mysql2-0.3.11 for inspection.
Results logged to /usr/local/rvm/gems/ruby-2.0.0-p195/gems/mysql2-0.3.11/ext/mysql2/gem_make.out

Wait. What? The mysql.h does exist (and is valid) in the /usr/local/mysql/include folder.
I, of course, realised that I had not set the --with-mysql-include folder and tried that along with some others.
Nope.
No luck.

I did some googling and the options where confusing and ultimately useless.
The most common options are:

• Install mysql server using homebrew. Wait. What?
• Setting env ARCHFLAGS="-arch x86_64" before running gem.
• Using bundle to create a .bundle/config file.
• Including every --with-mysql option available on the gem install.

I didn't reinstall mysql as that seemed like silly talk.
I did try the --with-mysql-config=/usr/local/mysql/bin/mysql_config option with no luck.
I had this tickling sensation in my brain that indicated that I was buggerising around in the wrong 'mode' and that I was missing something gob smackingly obvious.
But I persevered.

I tested the /usr/local/mysql/bin/mysql_config program and it's output was like this:

Usage: /usr/local/mysql/bin/mysql_config [OPTIONS]
Options:
        --cflags         [-I/usr/local/mysql/include  -Wno-unused-private-field -Os -g -fno-strict-aliasing -arch x86_64]
        --cxxflags       [-I/usr/local/mysql/include  -Wno-unused-private-field -Os -g -fno-strict-aliasing -arch x86_64]
        --include        [-I/usr/local/mysql/include]
        --libs           [-L/usr/local/mysql/lib -lmysqlclient]
        --libs_r         [-L/usr/local/mysql/lib -lmysqlclient_r]
        --plugindir      [/usr/local/mysql/lib/plugin]
        --socket         [/tmp/mysql.sock]
        --port           [0]
        --version        [5.6.11]
        --libmysqld-libs [-L/usr/local/mysql/lib -lmysqld]
        --variable=VAR   VAR is one of:
                pkgincludedir [/usr/local/mysql/include]
                pkglibdir     [/usr/local/mysql/lib]
                plugindir     [/usr/local/mysql/lib/plugin]

So I did some digging around in the gems folder and under the ext/mysql2 folder there was the mkmf.log file.
As I was stuffed up with flu, surrounded by used tissues and it was 3am it took me a little while to see the issue.
Then I saw it:

cc1: error: unrecognized command line option "-Wno-null-conversion"

It's cc1.
It's failing because it has been provided with a -W gcc option that isn't supported by the compiler.
I tried without luck to get "gem" and the "extconf.rb" to use the ENV['CC'] variable.
So using the gcc-4.2 compiler was the only option.
I checked the compiler version:

gcc -v
Using built-in specs.
Target: i686-apple-darwin11
Configured with: /private/var/tmp/llvmgcc42/llvmgcc42-2335.15~25/src/configure --disable-checking --enable-werror --prefix=/Developer/usr/llvm-gcc-4.2 --mandir=/share/man --enable-languages=c,objc,c++,obj-c++ --program-prefix=llvm- --program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ --with-slibdir=/usr/lib --build=i686-apple-darwin11 --enable-llvm=/private/var/tmp/llvmgcc42/llvmgcc42-2335.15~25/dst-llvmCore/Developer/usr/local --program-prefix=i686-apple-darwin11- --host=x86_64-apple-darwin11 --target=i686-apple-darwin11 --with-gxx-include-dir=/usr/include/c++/4.2.1
Thread model: posix
gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)

Then it struck me that I was an idiot.
I edited the /usr/local/mysql/bin/mysql_config file and removed the offending -W compiler options.
(It's around line 120 or so)
The original looks like this:

cflags="-I$pkgincludedir  -Wall -Wno-null-conversion -Wno-unused-private-field -Os -g -fno-strict-aliasing -DDBUG_OFF -arch x86_64 " #note: end space!
cxxflags="-I$pkgincludedir  -Wall -Wno-null-conversion -Wno-unused-private-field -Os -g -fno-strict-aliasing -DDBUG_OFF -arch x86_64 " #note: end space!

And the changed version:

cflags="-I$pkgincludedir  -Wall -Os -g -fno-strict-aliasing -DDBUG_OFF -arch x86_64 " #note: end space!
cxxflags="-I$pkgincludedir  -Wall -Os -g -fno-strict-aliasing -DDBUG_OFF -arch x86_64 " #note: end space!

Now the install works just peachy.

Oh. One last point.
I had to link the dylib to /usr/local/lib:

ln -s /usr/local/mysql/lib/libmysqlclient.18.dylib /usr/lib/libmysqlclient.18.dylib