gemでインストールを行おうとしたときに no such file to load -- mkmf (LoadError)


$ sudo gem install mongo_mapper
Building native extensions. This could take a while...
ERROR: Error installing mongo_mapper:
ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb
extconf.rb:36:in `require': no such file to load -- mkmf (LoadError)
from extconf.rb:36


$ sudo apt-get install ruby1.8-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
0 upgraded, 1 newly installed, 0 to remove and 127 not upgraded.
Need to get 577kB of archives.
After this operation, 1,634kB of additional disk space will be used.
Get:1 lucid/main ruby1.8-dev [577kB]
Fetched 577kB in 0s (1,013kB/s)
Selecting previously deselected package ruby1.8-dev.
(Reading database ... 70807 files and directories currently installed.)
Unpacking ruby1.8-dev (from .../ruby1.8-dev_1.8.7.249-2_i386.deb) ...
Setting up ruby1.8-dev ( ...