add builtin option to install licenses

Unless `meson.install_dependency_manifest()` is explicitly used, this
will cause a default implied one to be installed.

docs/markdown/Builtin-options.md

@@ -41,6 +41,7 @@ not be relied on, since they can be absolute paths in the following cases:
 | includedir                           | include       | Header file directory |
 | infodir                              | share/info    | Info page directory |
 | libdir                               | see below     | Library directory |
+| licensedir                           | see below     | Licenses directory (since 1.1.0)|
 | libexecdir                           | libexec       | Library executable directory |
 | localedir                            | share/locale  | Locale data directory |
 | localstatedir                        | var           | Localstate data directory |
@@ -61,6 +62,10 @@ different distributions have different defaults. Using a [cross
 file](Cross-compilation.md#defining-the-environment), particularly the
 paths section may be necessary.
 
+`licensedir` is empty by default. If set, it defines the default location
+to install a dependency manifest and project licenses. For more details,
+see [[meson.install_dependency_manifest]].
+
 ### Core options
 
 Options that are labeled "per machine" in the table are set per

docs/markdown/snippets/licensesdir_option.md

@@ -0,0 +1,4 @@
+## A new core directory option "licensedir" is available
+
+This will install a dependency manifest to the specified directory, if none
+is is explicitly set.

docs/yaml/builtins/meson.yaml

@@ -323,6 +323,10 @@ methods:
       If license files are defined as well, they will be copied next to the
       manifest and referenced in it.
 
+      If this function is not used, the builtin option `licensedir` can
+      be used to install the manifest to a given directory with the name
+      `depmf.json`.
+
     posargs:
       output_name:
         type: str

mesonbuild/backend/backends.py

@@ -1187,13 +1187,19 @@ def construct_target_rel_paths(self, t: T.Union[build.Target, build.CustomTarget
         return outputs
 
     def generate_depmf_install(self, d: InstallData) -> None:
-        if self.build.dep_manifest_name is None:
-            return
+        depmf_path = self.build.dep_manifest_name
+        if depmf_path is None:
+            option_dir = self.environment.coredata.get_option(OptionKey('licensedir'))
+            assert isinstance(option_dir, str), 'for mypy'
+            if option_dir:
+                depmf_path = os.path.join(option_dir, 'depmf.json')
+            else:
+                return
         ifilename = os.path.join(self.environment.get_build_dir(), 'depmf.json')
-        ofilename = os.path.join(self.environment.get_prefix(), self.build.dep_manifest_name)
-        odirname = os.path.join(self.environment.get_prefix(), os.path.dirname(self.build.dep_manifest_name))
-        out_name = os.path.join('{prefix}', self.build.dep_manifest_name)
-        out_dir = os.path.join('{prefix}', os.path.dirname(self.build.dep_manifest_name))
+        ofilename = os.path.join(self.environment.get_prefix(), depmf_path)
+        odirname = os.path.join(self.environment.get_prefix(), os.path.dirname(depmf_path))
+        out_name = os.path.join('{prefix}', depmf_path)
+        out_dir = os.path.join('{prefix}', os.path.dirname(depmf_path))
         mfobj = {'type': 'dependency manifest', 'version': '1.0',
                  'projects': {k: v.to_json() for k, v in self.build.dep_manifest.items()}}
         with open(ifilename, 'w', encoding='utf-8') as f:

mesonbuild/coredata.py

@@ -1197,6 +1197,7 @@ def add_to_argparse(self, name: str, parser: argparse.ArgumentParser, help_suffi
     (OptionKey('includedir'),      BuiltinOption(UserStringOption, 'Header file directory', 'include')),
     (OptionKey('infodir'),         BuiltinOption(UserStringOption, 'Info page directory', 'share/info')),
     (OptionKey('libdir'),          BuiltinOption(UserStringOption, 'Library directory', default_libdir())),
+    (OptionKey('licensedir'),      BuiltinOption(UserStringOption, 'Licenses directory', '')),
     (OptionKey('libexecdir'),      BuiltinOption(UserStringOption, 'Library executable directory', default_libexecdir())),
     (OptionKey('localedir'),       BuiltinOption(UserStringOption, 'Locale data directory', 'share/locale')),
     (OptionKey('localstatedir'),   BuiltinOption(UserStringOption, 'Localstate data directory', 'var')),

mesonbuild/utils/universal.py

@@ -2062,6 +2062,7 @@ class OptionType(enum.IntEnum):
     'includedir',
     'infodir',
     'libdir',
+    'licensedir',
     'libexecdir',
     'localedir',
     'localstatedir',