Ninja: Add option for parallel install

Adds the global property ``INSTALL_PARALLEL`` to enable a parallel install
target for Ninja.

Fixes: #25459

Help/generator/Ninja.rst

@@ -24,6 +24,13 @@ Builtin Targets
     The ``CMAKE_STRIP`` variable will contain the platform's ``strip`` utility, which
     removes symbols information from generated binaries.
 
+``install/parallel``
+
+  .. versionadded:: 3.30
+
+    Created only if the :prop_gbl:`INSTALL_PARALLEL` global property is ``ON``.
+    Runs the install step for each subdirectory independently and in parallel.
+
 For each subdirectory ``sub/dir`` of the project, additional targets
 are generated:
 

Help/manual/cmake-properties.7.rst

@@ -39,6 +39,7 @@ Properties of Global Scope
    /prop_gbl/GENERATOR_IS_MULTI_CONFIG
    /prop_gbl/GLOBAL_DEPENDS_DEBUG_MODE
    /prop_gbl/GLOBAL_DEPENDS_NO_CYCLES
+   /prop_gbl/INSTALL_PARALLEL
    /prop_gbl/IN_TRY_COMPILE
    /prop_gbl/JOB_POOLS
    /prop_gbl/PACKAGES_FOUND

Help/prop_gbl/INSTALL_PARALLEL.rst

@@ -0,0 +1,23 @@
+INSTALL_PARALLEL
+----------------
+
+.. versionadded:: 3.30
+
+Enables parallel installation option for the Ninja generator.
+
+When this property is ``ON``, ``install/local`` targets have the
+console pool disabled, allowing them to run concurrently.
+
+This property also provides the target ``install/parallel``, which has an
+explicit dependency on the ``install/local`` target for each subdirectory,
+recursing down the project.
+
+Setting this property has no affect on the behavior of ``cmake --install``.
+The install must be invoked by building the ``install/parallel`` target
+directly.
+
+Calls to :command:`install(CODE)` or :command:`install(SCRIPT)` might depend
+on actions performed by an earlier :command:`install` command in a different
+directory such as files installed or variable settings. If the project has
+such order-dependent installation logic, parallel installation should be
+not be enabled, in order to prevent possible race conditions.