Make installing non-existing subdirs a supported feature

install_subdir() with a non-existing subdir creates the directory in the
target directory. This seems like an implementation detail but is quite useful
to create new directories for e.g. configuration or plugins in the installed
locations.

git bisect says this started with 8fe8161014.
Let's add a test for it and document it to make this behavior official.

Limitation: it can only create at the install_dir location, trying to create
nested subdirectories does not work and indeed creates the wrong directory
structure. That is a bug that should be fixed separately:

        install_subdir('blah',
                        install_dir: get_option('prefix'))
        install_subdir('sub/foobar',
                        install_dir: get_option('prefix'))
        install_subdir('foo/baz',
                        install_dir: get_option('prefix'))

        $ tree ../_inst
        ../_inst
        ├── baz
        ├── blah
        └── foobar

Fixes #2904

docs/markdown/Reference-manual.md

@@ -1143,6 +1143,10 @@ Installs the entire given subdirectory and its contents from the
 source tree to the location specified by the keyword argument
 `install_dir`.
 
+If the subdirectory does not exist in the source tree, an empty directory is
+created in the specified location. *(since 0.45.0)* A newly created
+subdirectory may only be created in the keyword argument `install_dir`.
+
 The following keyword arguments are supported:
 
 - `exclude_files`: a list of file names that should not be installed.
@@ -1192,6 +1196,12 @@ share/
   file1
 ```
 
+`install_subdir('new_directory', install_dir : 'share')` creates
+```text
+share/
+  new_directory/
+```
+
 ### is_disabler()
 
 ``` meson

run_unittests.py

@@ -2414,6 +2414,7 @@ class AllPlatformTests(BasePlatformTests):
             'sub/sub1': 'share/sub1',
             'sub_elided': 'share',
             'nested_elided/sub': 'share',
+            'new_directory': 'share/new_directory',
         }
 
         self.assertEqual(len(intro), len(expected))

test cases/common/60 install subdir/meson.build

@@ -16,3 +16,6 @@ install_subdir('sub/sub1', install_dir : 'share')
 # strip_directory
 install_subdir('sub_elided', install_dir : 'share', strip_directory : true)
 install_subdir('nested_elided/sub', install_dir : 'share', strip_directory : true)
+
+# Create new empty directory that doesn't exist in the source tree
+install_subdir('new_directory', install_dir : 'share')

test cases/common/60 install subdir/test.json

@@ -11,6 +11,7 @@
     {"type": "file", "file": "usr/share/sub1/third.dat"},
     {"type": "file", "file": "usr/share/sub1/sub2/data2.dat"},
     {"type": "file", "file": "usr/share/sub2/one.dat"},
-    {"type": "file", "file": "usr/share/sub2/dircheck/excluded-three.dat"}
+    {"type": "file", "file": "usr/share/sub2/dircheck/excluded-three.dat"},
+    {"type": "dir", "file": "usr/share/new_directory"}
   ]
 }