Ninja: Support embedding of CMake as subninja project

Add a `CMAKE_NINJA_OUTPUT_PATH_PREFIX` variable.  When it is set, CMake
generates a `build.ninja` file suitable for embedding into another ninja
project potentially generated by an alien generator.

1 files changed, 27 insertions, 0 deletions

diff --git a/Help/variable/CMAKE_NINJA_OUTPUT_PATH_PREFIX.rst b/Help/variable/CMAKE_NINJA_OUTPUT_PATH_PREFIX.rst
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+++ b/Help/variable/CMAKE_NINJA_OUTPUT_PATH_PREFIX.rst
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+CMAKE_NINJA_OUTPUT_PATH_PREFIX
+------------------------------
+
+Set output files path prefix for the :generator:`Ninja` generator.
+
+Every output files listed in the generated ``build.ninja`` 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/build.ninja`` with a directive like
+this::
+
+  subninja sub/build.ninja
+
+The ``auto-regeneration`` rule in ``top-build-dir/build.ninja`` must have an
+order-only dependency on ``sub/build.ninja``.
+
+.. note::
+  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.