Taken directly from Stackoverflow (http://stackoverflow.com/a/17262871/80492)
...
What worked for me in Mountain Lion Rails install(using Homebrew and RVM) was editing /usr/local/Cellar/mysql/5.XX.XX/bin/mysql_config and removing
-Wno-null-conversion -Wno-unused-private-field from cxflags and cxflags options as follows:
Before:
cflags="-I$pkgincludedir -Wall -Wno-null-conversion -Wno-unused-private-field -Os -g -fno-strict-aliasing -DDBUG_OFF " #note: end space!
cxxflags="-I$pkgincludedir -Wall -Wno-null-conversion -Wno-unused-private-field -Os -g -fno-strict-aliasing -DDBUG_OFF " #note: end space!
After:
cflags="-I$pkgincludedir -Wall -Os -g -fno-strict-aliasing -DDBUG_OFF " #note: end space!
cxxflags="-I$pkgincludedir -Wall -Os -g -fno-strict-aliasing -DDBUG_OFF " #note: end space!
After that gem install mysql2 proceeded without hickups
