Make install targets optional in CMake

[arludwig]

I am integrating SimpleBLE in a project as submodule, to be compiled and built along with the project's code. I also use install rules for my project.

Now I have the problem, that SimpleBLE's install rules are also added and executed, which is not what I want or need. Many projects offer some sort of option to disable install rules. For SimpleBLE this could look like

option(INSTALL_SimpleBLE "Disable installing" ON)

that can be set before including

set(INSTALL_SimpleBLE OFF)
add_subdirectory("SimpleBLE/simpleble")

My current workaround is to use the EXCLUDE_FROM_ALL flag for add_subdirectory()

add_subdirectory("SimpleBLE/simpleble" EXCLUDE_FROM_ALL)

which works fine for me. But technically it does way more than just disabling installs, so I think it would be nice for SimpleBLE to offer such option.