[CMake] Building Multiple projects from a single CMakeList

[ML]

Hi All,

So my learning is going ok so far.

My question right now is how might I check for the existence of a  
previously build library. If it exists do nothing, if it does not  
exist build it.

My Example:
1. if libexpat.a exists in bin/ do nothing
2. if libexpat.a does not exist, my main project needs it, so build it.

Can I have a target called 'expat' and variable for the expat source  
files, etc and another add_executable?

Does anyone have a template?

Best,

-Jason