Set output files path prefix for the Ninja generator.

Every output files listed in the generated will be prefixed by the contents of this variable (a trailing slash is appended if missing). This is useful when the generated ninja file is meant to be embedded as a subninja file into a super ninja project. For example, a ninja build file generated with a command like:

cd top-build-dir/sub &&
cmake -G Ninja -DCMAKE_NINJA_OUTPUT_PATH_PREFIX=sub/ path/to/source

can be embedded in top-build-dir/ with a directive like this:

subninja sub/

The auto-regeneration rule in top-build-dir/ must have an order-only dependency on sub/


When CMAKE_NINJA_OUTPUT_PATH_PREFIX is set, the project generated by CMake cannot be used as a standalone project. No default targets are specified.