Macros can be hard to manipulate and vary among different compilers and
platforms. For example, GNU Hurd has `__MACH__` defined but not
`__APPLE__`.
Let CMake choose them during configuration.
Revert needed, since otherwise there is no way to do automatic sorting
of includes.
Also reverted change to the docs, as leaving it would make incorrect
docs.
In case of conflicts, includes were sorted according to the coding
standards from #3839.
This reverts commit b4a9f04f92.
This reverts commit 5921122960.
