Follow up commit 0761186949 (project: Add variable
CMAKE_PROJECT_INCLUDE_BEFORE, 2019-03-28, v3.15.0-rc1~294^2) with a
project-specific variant. This variable will be used similar to the
already existing `CMAKE_PROJECT_INCLUDE_BEFORE` and
`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE` variables.
Fixes: #19854