CMake Build System Breaks Containment

A few build system comments (because what's there is not subprojectable nor selfcontained). I'm sure most of these were on your list, but just so you have them for when you want to mess with the build system: 1. It's generally not a good practice to overwrite `CMAKE_<LANG>_FLAGS` (or really any `CMAKE_XYZ` flags, with a few exceptions as long as you put them back). These are global variables, so it break containment and will make debugging later much more difficult. The only place I see you using this right now is `OpenMP`, see next comments. 2. Modern CMake prefers `TARGETs` over manual including/linking various flags and libraries. The target you want here is `OpenMP::OpenMP_CXX`, that will include all compiler flags and libraries required to use OpenMP on your platform and it will allow for contained linkage (e.g. `target_link_libraries(my_lib PUBLIC OpenMP::OpenMP_CXX)` will only affect my_lib and libraries that link to it. It will also make packaging much easier) 3. Same with `LAPACK` - link_libraries is global, always prefer target_link_libraries and `TARGETs`. Unfortunately, BLAS/LAPACK targets weren't added until a very recent CMake. However, I maintain a set of very robust CMake modules to handle BLAS/LAPACK discovery (they're also more robust than the Kitware Find{BLAS,LAPACK}.cmake). The repo is [here](https://github.com/wavefunction91/linalg-cmake-modules) and an example usage is [here](https://github.com/wavefunction91/GauXC/blob/master/cmake/gauxc-linalg-modules.cmake). You don't actually need to change `find_package(LAPACK)`, the same semantics apply. 4. Armadillo actually has a [cmake module](https://cmake.org/cmake/help/latest/module/FindArmadillo.html). Unfortunately it doesn't use `TARGETs` :disappointed: , but it could be made to pretty easily. I'll open a ticket with Kitware. Again, prefer `target_linclude_directories` over `include_directories` as the latter is global. 5. What is `optim`? It should be packaged if its not already. If it's not, let me know, and I might be able to whip something up quickly