[CMake] Recursively include target include directories only

[Dan Liew]

> This is the approach that the LLVM project uses which is very simple
> and very clean (take a look at the sources). This also works very well
> for installing your header files, the contents of ${CMAKE_SOURCE_DIR}
> just need to be copied into /usr/include .

Oops I meant to say the contents of

${CMAKE_SOURCE_DIR}/include

and

${CMAKE_BINARY_DIR}/include

can be copied into /usr/include.