build: inform the build of what generates version.h

Without this, Ninja requires a two-step process because it doesn't know
what generates version.h it sees from its depfile support and when it
changes in one run, it has already pruned the dependent nodes from its
internal graph and requires a second run to catch it.

version.cmake

@@ -11,12 +11,22 @@ if ("$Format:$" STREQUAL "")
   write_static_version_header("release")
 elseif (GIT_FOUND OR Git_FOUND)
   message(STATUS "Found Git: ${GIT_EXECUTABLE}")
-  add_custom_target(version ALL
+  set(extra_output)
+  if (CMAKE_GENERATOR MATCHES "Ninja")
+    # Ninja will not rerun the command every time if the file doesn't change,
+    # so inject this bogus output so that it always runs.
+    set(extra_output "${CMAKE_SOURCE_DIR}/.force-git-version-check")
+  endif ()
+  add_custom_command(
+    OUTPUT            "${CMAKE_BINARY_DIR}/version/version.h"
+                      ${extra_output}
     COMMAND           "${CMAKE_COMMAND}"
                       "-D" "GIT=${GIT_EXECUTABLE}"
                       "-D" "TO=${CMAKE_BINARY_DIR}/version/version.h"
                       "-P" "src/version.cmake"
     WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}")
+  add_custom_target(version ALL
+    DEPENDS "${CMAKE_BINARY_DIR}/version/version.h")
 else()
   message(STATUS "WARNING: Git was not found!")
   write_static_version_header("unknown")