ExternalProject: Add `INSTALL_BYPRODUCTS` option

Add an `INSTALL_BYPRODUCTS` option to `ExternalProject_Add` that can
be used to declare that files are `BYPRODUCTS` of the ExternalProject
install step.

This is often required by the Ninja generator to explicitly declare
dependencies. Previously, many users used `BUILD_BYPRODUCTS`, even if
their files were created by the install step, not the build step.

This commit essentially just copies the code for `BUILD_BYPRODUCTS`.

Fixes: #24120
Fixes: #23056

1 files changed, 18 insertions, 0 deletions

diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index 4af0952..9fecd8f 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -664,6 +664,17 @@ External Project Definition
       supported). Passing an empty string as the ``<cmd>`` makes the install
       step do nothing.
 
+    ``INSTALL_BYPRODUCTS <file>...``
+      .. versionadded:: 3.26
+
+      Specifies files that will be generated by the install command but which
+      might or might not have their modification time updated by subsequent
+      installs. This may also be required to explicitly declare dependencies
+      when using the :generator:`Ninja` generator.
+      These ultimately get passed through as ``BYPRODUCTS`` to the
+      install step's own underlying call to :command:`add_custom_command`, which
+      has additional documentation.
+
     .. note::
       If the :envvar:`CMAKE_INSTALL_MODE` environment variable is set when the
       main project is built, it will only have an effect if the following
@@ -3852,6 +3863,11 @@ function(_ep_add_install_command name)
     set(always 0)
   endif()
 
+  get_property(install_byproducts
+    TARGET ${name}
+    PROPERTY _EP_INSTALL_BYPRODUCTS
+  )
+
   set(__cmdQuoted)
   foreach(__item IN LISTS cmd)
     string(APPEND __cmdQuoted " [==[${__item}]==]")
@@ -3860,6 +3876,7 @@ function(_ep_add_install_command name)
     ExternalProject_Add_Step(${name} install
       INDEPENDENT FALSE
       COMMAND ${__cmdQuoted}
+      BYPRODUCTS \${install_byproducts}
       WORKING_DIRECTORY \${binary_dir}
       DEPENDEES build
       ALWAYS \${always}
@@ -4087,6 +4104,7 @@ function(ExternalProject_Add name)
     # Install step options
     #
     INSTALL_COMMAND
+    INSTALL_BYPRODUCTS
     #
     # Test step options
     #