Make Vala builds efficient with Ninja

cmake/ValaPrecompile.cmake

@@ -40,18 +40,25 @@ macro (_vala_parse_source_path)
 	set (output_c    "${directory}/${without_extension}.c")
 endmacro ()
 
-# XXX: it would be best to have it working without touching the vapi (which
-#   forces rebuilds even if the API doesn't change) but it appears this cannot
-#   be done in CMake (at least with make, with ninja it seems to be possible
-#   via stamps and BYPRODUCTS)
-# TODO: check ${CMAKE_GENERATOR} and try to do better -> OUTPUT and "touch"
-#   a stamp, use BYPRODUCTS instead, ... maybe make a target for all stamps,
-#   then DEPEND on that target when precompiling.  This should be enough.
 function (_vala_make_fast file)
 	_vala_parse_source_path ()
-	add_custom_command (OUTPUT "${output_vapi}"
+
+	# See `cmake --help-policy CMP0058` for a rather nice explanation.
+	# TODO: it says we can fix this situation even for non-Ninja
+	#   (rebuilds are forced even when the external API doesn't change),
+	#   just make sure to only use BYPRODUCTS when it's supported by CMake
+	set (stamp "${output_vapi}")
+	set (byproducts)
+	if ("${CMAKE_GENERATOR}" STREQUAL Ninja)
+		set (stamp "${output_vapi}.stamp")
+		set (byproducts BYPRODUCTS "${output_vapi}")
+		set (stamp_target ${stamp_target} "${stamp}" PARENT_SCOPE)
+	endif ()
+
+	add_custom_command (OUTPUT "${stamp}"
+		${byproducts}
 		COMMAND ${VALA_COMPILER} "${file}" "--fast-vapi=${output_vapi}"
-		COMMAND ${CMAKE_COMMAND} -E touch "${output_vapi}"
+		COMMAND ${CMAKE_COMMAND} -E touch "${stamp}"
 		DEPENDS "${file}"
 		COMMENT "Generating a fast vapi for ${basename}" VERBATIM)
 	set (fast_vapi_deps ${fast_vapi_deps} "${output_vapi}" PARENT_SCOPE)
@@ -69,13 +76,18 @@ function (_vala_make_c file)
 	add_custom_command (OUTPUT "${output_c}"
 		COMMAND ${VALA_COMPILER} "${file}" -C ${compiler_args} ${fast_vapis}
 		COMMAND ${CMAKE_COMMAND} -E touch "${output_c}"
-		DEPENDS "${file}" ${fast_vapi_deps} ${vapis}
+		DEPENDS "${file}" ${fast_vapi_deps} ${vapis} ${stamp_target}
 		WORKING_DIRECTORY "${directory}"
 		COMMENT "Precompiling ${basename}" VERBATIM)
 	set (c_outputs ${c_outputs} "${output_c}" PARENT_SCOPE)
 endfunction ()
 
+set (_vala_counter 0)
 function (vala_precompile)
+	# So that the user doesn't need to name any targets, we can do it ourselves
+	math (EXPR counter "${_vala_counter} + 1")
+	set (_vala_counter "${counter}" PARENT_SCOPE)
+
 	set (_single_value DIRECTORY OUTPUTS HEADER VAPI SYMBOLS)
 	set (_multi_value PACKAGES OPTIONS)
 	cmake_parse_arguments (arg "" "${_single_value}" "${_multi_value}" ${ARGN})
@@ -105,9 +117,14 @@ function (vala_precompile)
 
 	set (fast_vapi_deps)
 	set (fast_vapi_args)
+	set (stamp_target)
 	foreach (_source ${sources})
 		_vala_make_fast ("${_source}")
 	endforeach ()
+	if (stamp_target)
+		add_custom_target ("vala_fast_vapis_${counter}" DEPENDS ${stamp_target})
+		set (stamp_target "vala_fast_vapis_${counter}")
+	endif ()
 
 	set (compiler_args)
 	foreach (_pkg ${arg_PACKAGES})