Document and add a cmake guard for minimum compiler versions

[paul0403]

We use c++20 across the board.
However, the clang compiler only has partial support for c++20 in some older versions.

We should clearly document the minimum clang and gcc version required to build catalyst from source.

We should also add a guard for it in cmake, to terminate the build at config time if minimum compiler versions are not met on the system.
For how to do it in cmake, see https://stackoverflow.com/questions/14933172/how-can-i-add-a-minimum-compiler-version-requisite

if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
    # require at least gcc 4.8
    if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
        message(FATAL_ERROR "GCC version must be at least 4.8!")
    endif()
elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
    # require at least clang 3.2
    if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.2)
        message(FATAL_ERROR "Clang version must be at least 3.2!")
    endif()
else()
    message(WARNING "You are using an unsupported compiler! Compilation has only been tested with Clang and GCC.")
endif()