[CMake] get the relative path of current directory

[blinkeye]

Use case:

You start developing src/prototype1 which implements feature X. Next, you copy
src/prototype1 to src/prototype2 and implement feature Y or enhance feature X.
You don't branch because you're constantly comparing these prototypes against
each others (say for speed).

To make this really easy I need a way of getting the ${RELATIVE_SOURCE_DIR} of
such a prototype so I can copy src/prototype1 to src/prototypeX without
adjusting src/prototypeX/CMakeLists.txt:

set( EXECUTABLE_OUTPUT_PATH ${PROJECT_BINARY_DIR}/bin/${RELATIVE_SOURCE_DIR} )

So, the only thing one needs to adjust is the project's top CMakeLists.txt and
add the new prototype's source dir to the SUBDIRS directive.

Is this possible?