When we explain in add_definitions documentation that it is intended
only for preprocessor definitions, link to add_compile_options for
adding other flags. Also explicitly mention that the order of
add_definitions calls with respect to target creation does not matter.
This differs from the behavior now explicitly stated in the
documentation of add_compile_options.
This command does not support generator expressions. The documentation
was mistakenly extended to claim it in commit v3.0.0-rc1~60^2~3 (Help: Mark
up the buildsystem commands documentation, 2014-02-03).