title: Multiple source directories for one executable with CMake
body: <p>I want my source organised in a number of subdirectories but be able to create a single executable without having to build a library for each subdirectory. Can CMake do this? Something like:</p>
  
  <p>ADD_EXECUTABLE(foo a/main.cpp a/other.cpp b/another.cpp)</p>
  
  <p>Is it that simple? With the / working as a directory separator regardless of platform?</p>