As of CMake 3.0 there is a special syntax for block comments which start with #[[ and ends with ]] at the end of block comment. See CMake documentation for further explanation.
You can also put a number of equal signs between the brackets as long as the number between the opening [[ and closing ]] are both the same. Ex.
#[===[
hello
]] <- this is not the closing
]===]
# ^ this is the closing
Some editors and IDEs do not support it yet.