CMake Nightly Date Stamp
add_definitions: Don't document genex support.

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).
Merge topic 'add_definitions-no-genex' into next

61839b15 add_definitions: Don't document genex support.
115ffb0e CMake Nightly Date Stamp