Several minor fixes for the cmake system

[anderkve]

This PR collects a number of tiny fixes to the cmake system, to correct some small bugs and typos, and improve some error messages to make them more useful.

Bugs fixed:

• cmake/warnings.cmake: set_compiler_warning("error") now passes CMAKE_CXX_FLAGS so -DWERROR=ON actually adds -Werror.
• cmake/optional.cmake: ${CoulourReset} typo corrected to ${ColourReset} in the C-MPI-without-C++-MPI warning.

Improved messages:

• CMakeLists.txt: minimum-compiler FATAL_ERROR now mentions clang++ and AppleClang minimums alongside g++ and icpc.
• CMakeLists.txt: Eigen3 FATAL_ERROR now points users at -DEIGEN3_INCLUDE_DIR=/path/... and the EIGEN3_ROOT env var as alternatives to installing.
• cmake/optional.cmake: ROOT path now distinguishes ROOTSYS unset (suggests sourcing thisroot.sh) from "found ROOTSYS but no ROOT 6 there" (echoes the path).

Documentation fixes:

• README.md, README_OSX.md: CMAKE_FLAGS.md references corrected to BUILD_OPTIONS.md.
• README.md: dropped future from the required-Python-modules list (it's not actually checked).

Typos, etc.:

• CMakeLists.txt: build-type message no longer wraps undefined colour vars (printed before utilities.cmake is included). Per your request, kept in place — colour stripped instead of moved.
• CMakeLists.txt: Cmake 3.0 → CMake 3.0 in CMP0048 comment.
• CMakeLists.txt: Python_ADDITIONAL_VERSIONS extended to 3.12, 3.13.
• cmake/optional.cmake: removed unreachable LAPACK message after the FATAL_ERROR.