These are minor cleanups that do not affect the behaviour of the script.
In particular, I removed most `DESTINATION` arguments from `install()`,
since they are automatically deduced based on the type of file to be
installed.
In #662 I also added doxygen_add_docs and gtest_discover_tests, but they
require CMake version 3.9 and 3.10 respectively, and I think that the
advantages they provide are not worth dropping support for older versions
