Necessary especially for multi-configuration generators

split boost libs / additional libs: Necessary for multi-config generators (continued in next commits)

to avoid ambiguities while allowing for concise file names

and omit 'BORNAGAIN_' so that I can use certain shell aliases across
projects.

Treat it as any other 3rd-party dependence

this requires some ad-hoc code to work with both the C and the CPP
version of libcerf.

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

Wuttke, Joachim authored 4 years ago and Wuttke, Joachim committed 4 years ago

Our patch, https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4934,
is expected to be released with CMake 3.19. Therefore we shall removed
FindTIFF.cmake from the BornAgain sources, once our minimum required
CMake version has been incremented accordingly.