Add cppcheck

This patch adds cppcheck to Externals. cppcheck is a static code check tool. It seems very useful and has already been used to find bugs in other projects.

cppcheck homepage:
http://cppcheck.sourceforge.net/

Having this kind of tool in our SVN is a good thing:
- it is easy for people to compile and take into use
- we can require people to use it (no additional downloads etc)
- we can automate use of it later?
- it is small so no much overhead

This patch is the 1.24 version of the cppcheck codes + visual studio 2003 project files I created.