Install ibm_db on Ruby-mingw32

Here are the steps:

1. Install mingw compiled Ruby from http://rubyinstaller.org/download.html
2. Install Ruby Development Kit from http://rubyinstaller.org/download.html
3. follow the instruction of Development Kit
4. If you install IBM DB2 client in "c:\Program Files", you can do like this:

set IBM_DB_LIB=%ProgramFiles%/IBM/SQLLIB/LIB
set IBM_DB_INCLUDE=%ProgramFiles%/IBM/SQLLIB/INCLUDE
gem install ibm_db

Because there is a space in "Program Files", adding quotes or escaping the space like "\ " do not work for me.