WIP: Add a summary() function for configuration summarization

This has been requested multiple times in the mesa/xorg community, and
apparently in the GStreamer community as well.

This function can be called one per project (so subprojects can call it
as well), and takes up to a single positional argument, and an unlimited
number of free form keyword arguments, the values of which must be
dictionaries. At the end of the configuration phase all of the
information passed to the summary function is printed, separating the
main project from each subproject, and printing the group information.

For example:

```meson
sec1 = {'driver' : 'foobar', 'OS' : 'Linux', 'API' : 1.7}
...
sec2 = {'driver' : 'dive comp', 'OS' : 'Minix', 'API' : 1.1.2}
...
sec3 = {'with' : {'mesa' : true, 'gbm' : false}}

summary(
  {'Backend' : 'OpenGL'},
  Server : sec1,
  Client : sec2,
  Misc : sec3,
)
```

Which would print something like:

```txt
Configuration Summary:
  Backend = OpenGL
  Server:
    driver = foobar
    OS = Linux
    API = 1.7
  Client:
    driver = dive comp
    OS = Minix
    API = 1.1.2
  Misc:
    with = {'mesa : true, 'gbm' : false}
```

Fixes mesonbuild#757

mesonbuild/interpreter.py

@@ -1905,6 +1905,7 @@ def __init__(self, build, backend=None, subproject='', subdir='', subproject_dir
         self.coredata = self.environment.get_coredata()
         self.backend = backend
         self.subproject = subproject
+        self.summary = {}
         if modules is None:
             self.modules = {}
         else:
@@ -2009,6 +2010,7 @@ def build_func_dict(self):
                            'subdir': self.func_subdir,
                            'subdir_done': self.func_subdir_done,
                            'subproject': self.func_subproject,
+                           'summary': self.func_summary,
                            'shared_library': self.func_shared_lib,
                            'shared_module': self.func_shared_module,
                            'static_library': self.func_static_lib,
@@ -2268,7 +2270,7 @@ def func_option(self, nodes, args, kwargs):
     @stringArgs
     def func_subproject(self, nodes, args, kwargs):
         if len(args) != 1:
-            raise InterpreterException('Subproject takes exactly one argument')
+                raise InterpreterException('Subproject takes exactly one argument')
         dirname = args[0]
         return self.do_subproject(dirname, kwargs)
 
@@ -2361,6 +2363,7 @@ def do_subproject(self, dirname, kwargs):
         self.build_def_files += subi.build_def_files
         self.build.merge(subi.build)
         self.build.subprojects[dirname] = subi.project_version
+        self.summary.update(subi.summary)
         return self.subprojects[dirname]
 
     def get_option_internal(self, optname):
@@ -2603,6 +2606,54 @@ def func_message(self, node, args, kwargs):
         argstr = self.get_message_string_arg(node)
         mlog.log(mlog.bold('Message:'), argstr)
 
+    @FeatureNew('summary', '0.50.0')
+    def func_summary(self, node, args, kwargs):
+        if self.subproject in self.summary:
+            raise InterpreterException('Summary can only be called once per project.')
+
+        if len(args) > 1:
+            raise InterpreterException('Summary accepts zero or one argument.')
+        try:
+            # To convert None to a dict
+            arg = args[0] or {}
+        except IndexError:
+            arg = {}
+
+        if not isinstance(arg, dict):
+            raise InterpreterException('Summary argument must be a dictionary.')
+        if not all(isinstance(a, dict) for a in kwargs.values()):
+            raise InterpreterException('Summary kwargs must be dictionaries.')
+
+        self.summary[self.subproject] = (arg, kwargs)
+
+    def _print_summary(self):
+        """Print the configuration summary.
+
+        Called after all projects and subprojects are configured and used to
+        print all of the configuration at once.
+        """
+        mlog.log('')
+        mlog.log(mlog.bold('Configuration Summary:'))
+
+        def printer(arg, kwargs):
+            for k, v in arg.items():
+                mlog.log(' ', k, '=', v)
+            for s, t in kwargs.items():
+                mlog.log(' ', mlog.bold(s + ':'))
+                for k, v in t.items():
+                    mlog.log('   ', k, '=', v)
+            mlog.log('')
+
+        for project, (arg, kwargs) in sorted(self.summary.items()):
+            if project == '':
+                continue
+            mlog.log('', mlog.bold('Subproject : ' + project))
+            printer(arg, kwargs)
+
+        if self.summary.get(''):
+            mlog.log('', mlog.bold('Main Project'))
+            printer(*self.summary[''])
+
     @FeatureNew('warning', '0.44.0')
     @noKwargs
     def func_warning(self, node, args, kwargs):
@@ -3874,6 +3925,8 @@ def run(self):
         FeatureDeprecated.report(self.subproject)
         if not self.is_subproject():
             self.print_extra_warnings()
+        if self.subproject == '':
+            self._print_summary()
 
     def print_extra_warnings(self):
         for c in self.build.compilers.values():