cxxopts: make install optional

[shtrophic]

When using a header-only library such as cxxopts, it shouldn't be required for it to be installed to the system. I primarily oriented myself wrt. the header-only option of fmt.wrap.

[jpakkane]

meson install itself already has ways to not install parts of either by using --skip-subprojects or --tags. It is better to use those than implement custom code that does the same thing in every project.

[shtrophic]

... or to have the ability to declare dependencies as private in the first place.
The issue I see here is that adding flags requires the user of the software to invoke meson correctly, while configuring the project in meson.build is something the developer can do as he intends.