now inserted in ASSERT macro

Extend them to CMakeLists.txt,
and bundle the checks in one CMake module.

cmake/bornagain/modules -> cmake/modules
cmake/bornagain/scripts -> cmake/configurables
cmake/generic/modules   -> cmake/extensions

As recommended by
https://google.github.io/styleguide/cppguide.html#The__define_Guard

The following fix is necessary to ensure that the Boost installation as
found by CMake will be used, instead of the one that is installed in
/usr/include.