Installation destination of config files is missing a final path component

[eyalroz]

When installing a CMake package config file, the target directory is typically:

 ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}/

thus, for example, if we are installing project foo to /usr/local, we will put the config files under

/usr/local/cmake/foo/

this is not just an aesthetic idiom: When we find_package() with /usr/local in the CMAKE_PREFIX_PATH, CMake will search for the config files in that directory. It will not search in /usr/local/cmake itself - which is, unfortunately, where blitz currently puts its config files during installation.

So, let's add the missing path component.

PS - this format is also ok:

 ${CMAKE_INSTALL_LIBDIR}/${PROJECT_NAME}/cmake/

but it's more of a change.