[CMake] install targets

[Rolf Eike Beer]

> I have split my code up into two dirs, "source" and "tests".  The
> CMakeLists.txt in the source dir does a SUBDIRS to
> ${PROJECT_SOURCE_DIR}/../tests to get tests.

Reading the documentation may help:

subdirs
 Deprecated. Use the add_subdirectory() command instead.

HTH,

Eike