Implement recursive CMake.

Most CMake build systems put CMakeLists.txt files inside source directories where 
items need to get built. This change follows that convention by adding a new 
runtime/src/CMakeLists.txt file. An additional benefit is this helps logically 
seperate configuring with building as well. This change is mostly just copying and 
pasting the bottom half of runtime/CMakeLists.txt into runtime/src/CMakeLists.txt, 
but a few changes had to be made to get it to work. Most of those changes were to 
directory prefixes.

Differential Revision: http://reviews.llvm.org/D10344

llvm-svn: 239542