Fancier CMake config file

[jacobwilliams]

Update the CMake config file to enable more control of the build.

[jacobwilliams]

@japaf Do you have any interest in contributing your CMake updates to the main branch? We have a basic CMake configuration file now. I see that you also added one in your fork.

[japaf]

I am no CMake expert. The CMake config in my fork is largely based on https://cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file.

I needed a way to install the library such that it can be found by other CMake projects using find_package(). It installs shared library to <prefix>/lib/bspline, cmake config files to <prefix>/lib/cmake/bspline, .mod files to <prefix>/include/bspline, and pkgconfig files to <prefix>/lib/pkgconfig, where <prefix> is /usr/local by default, but can be overridden during compilation. However, I did not test it on Windows.

If that is something, you are interested in, I can create a pull request.