Not all consumers of GSL automatically want to have this install logic.

It's good practice to gate install logic behind an option.
For an example look at magic_enum:
https://github.com/Neargye/magic_enum/blob/master/CMakeLists.txt

If the client wants to install GSL they still can. But they should ask
for it by overriding GSL_INSTALL.

added nl@eof