build: Fix install target path

Use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR when defining
the path for the source files to be installed.

CMAKE_SOURCE_DIR always points at the directory containing the root
CMakeLists.txt, while CMAKE_CURRENT_SOURCE_DIR points at the directory
containing the current project's CMakeLists.txt. These are normally
identical, except when another CMake project includes this project via
add_subdirectory() --- in that case, CMAKE_CURRENT_SOURCE_DIR is the
right variable to use when composing paths to files within the current
project tree.

Change-Id: I4bdcc3bf538e96c43ae5f4ff6758cf45992442e0