Install compiler
apt-get install build-essential
On a fresh ubuntu 12.04 setup you will need to install some additional packages
apt-get install python2.7-dev
apt-get install libxml2-dev
apt-get install swig
Compile and install libSBML
Go to the directory where you have exctracted libsbml source and run these 3 commands
./configure --with-python
make
make install