build-sys: fix invalid args detected by meson 0.42 (systemd#6561)

some run_target() calls were using params from custom_target()

example message:
WARNING: Passed invalid keyword argument "input". This will become a hard error in the future.

New way to call targets:
ninja man/man
ninja man/html
ninja man/update-man-rules

man/meson.build

@@ -170,10 +170,10 @@ man = custom_target(
         depends : man_pages,
         command : ['echo'])
 
-html = run_target(
+html = custom_target(
         'html',
-        depends : html_pages,
         output : 'html',
+        depends : html_pages,
         command : ['echo'])
 
 run_target(
@@ -191,8 +191,9 @@ run_target(
 ############################################################
 
 if git.found()
-        run_target(
+        custom_target(
                 'update-man-rules',
+                output : 'update-man-rules',
                 # slightly strange syntax because of
                 # https://github.com/mesonbuild/meson/issues/1643
                 # and https://github.com/mesonbuild/meson/issues/1512

meson.build

@@ -2344,13 +2344,13 @@ if git.found()
                  ':/*.[ch]'])
         all_files = files(all_files.stdout().split())
 
-        run_target(
+        custom_target(
                 'tags',
-                input : all_files,
+                output : 'tags',
                 command : ['env', 'etags', '-o', '@0@/TAGS'.format(meson.source_root())] + all_files)
-        run_target(
+        custom_target(
                 'ctags',
-                input : all_files,
+                output : 'ctags',
                 command : ['env', 'ctags', '-o', '@0@/tags'.format(meson.source_root())] + all_files)
 endif