CMakeLists.txt files consistency

Both the [math CMakeLists.txt](https://github.com/boostorg/math/blob/c114cf4a6ae958ce87ed6db4d55c1db4ec0a9c74/CMakeLists.txt) and [mp11 CMakeLists.txt](https://github.com/boostorg/mp11/blob/c3eba6ac6be2e21af33a1b2ec97634cd01bcc447/CMakeLists.txt) contain the super project version as a literal like:
`project(boost_mp11 VERSION 1.92.0 LANGUAGES CXX)`
while all other libs do it like:
`project(boost_accumulators VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)`

Any particular reason why those 2 libs deviate from the general pattern ?
Otherwise there is a small improvement in the consistency of the CMakeLists.txt files to make.