Later on, we will move detection of required libraries, library
directories as well as include directories into a separate
CMakeLists.txt file inside of the source directory. Obviously, we want
to avoid duplication here regarding these parameters.

To prepare for the split, put the parameters into three variables
LIBGIT2_LIBS, LIBGIT2_LIBDIRS and LIBGIT2_INCLUDES, tracking the
required libraries, linking directory as well as include directories.
These variables can later be exported into the parent scope from inside
of the source build instructions, making them readily available for the
other subdirectories.