I'm try to install the SQLite gem on a Fedora 9 Linux box with Ruby 1.8.6, Rails 2.2.2, gem 1.3, and sqlite-3.5.9. Here's the command I'm running and its results:
sudo gem install sqlite3-ruby
Building native extensions. This could take a while...
ERROR: Error installing sqlite3-ruby:
ERROR: Failed to build gem native extension.
/usr/bin/ruby extconf.rb install sqlite3-ruby
can't find header files for ruby.
Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.2.4 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.2.4/ext/sqlite3_api/gem_make.out
The SQLite RubyGem isn't actually a *Ruby*Gem, it's a "*C*Gem", IOW it's written in C. This means it has to be compiled and linked to the Ruby interpreter when you install it and in order to do that it needs the C header files for the Ruby interpreter.
If you compile Ruby yourself, those header files will be installed automatically, however, in RedHat-ish systems, such header files are usually packaged in a seperate package, called
<whatever>-dev. So, in this case you will need to install the
ruby-dev package and possibly the
libsqlite3-dev (Ubuntu) or
sqlite-devel (Fedora) package as well.
However, you might be better off just installing your Operating System's pre-packaged
libsqlite3-ruby package, that way all the dependencies are automatically satisfied.
(Note: all package names pulled out of thin air, might be different on your system.)