feat: install cmake config module in arch-indep location

[pinotree]

toml11 is an header-only library, and the exported cmake target for it only points to the headers; because of this, it seems a good idea to install the cmake config module in an architecture-independent location for cmake, rather than under a library (architecture-dependent) location.

Doing so is easy:

• tweak TOML11_INSTALL_CMAKE_DIR to point to a cmake subdirectory of the shared dataroot location (typically $prefix/share)
• pass ARCH_INDEPENDENT argument to write_basic_package_version_file(): this is because the cmake config module generated by default contains the architecture bits check (32bit vs 64bit vs ...); since it does not matter in this case, make the config module arch-independent

[ToruNiina]

Thank you for the suggestion!

But, actually toml11 optionally supports precompiled library (libtoml11.a) that includes some deinline-ized functions and explicit specialization of frequently used functions (e.g. toml::parse<toml::type_config>). When -DTOML11_PRECOMPILE=ON, cmake will build binary and install it.
Since CMake says ARCH_INDEPENDENT is intended for packages with no binaries, it seems to be better not to use it in that case.

We can check TOML11_PRECOMPILE variable in cmake, but it makes script a bit complicated and harder to maintain.
I looked some of the libraries that supports both header-only and compiled modes, and many of them just uses LIBDIR in both modes (though I don't know they actually intended to do that). I will also take the same approach here.
So, I am sorry but I am closing this. I appreciate your cooperation. If you find anything else, please feel free to send it.