interpreter: Automatically add 'c' to languages when 'vala' is used

This is so dumb, we can just insert C for you without you having to know
that you're using C under the hood. This is nicer because:
1) Meson doesn't make the user add a language they're not explicitly
   using
2) If there was ever an implementaiton of Vala that didn't use C as
   it's assembly language, this wouldn't make any sense.

docs/markdown/snippets/vala-no-longer-requires-c.md

@@ -0,0 +1,4 @@
+## Using Vala no longer requires C in the project languages
+
+Meson will now add C automatically. Since the use of C is an implementation
+detail of Vala, Meson shouldn't require users to add it.

mesonbuild/interpreter/interpreter.py

@@ -1187,13 +1187,12 @@ external dependencies (including libraries) must go to "dependencies".''')
             return False
         return should
 
-    def add_languages_for(self, args, required, for_machine: MachineChoice):
+    def add_languages_for(self, args: T.List[str], required: bool, for_machine: MachineChoice) -> None:
         args = [a.lower() for a in args]
         langs = set(self.coredata.compilers[for_machine].keys())
         langs.update(args)
-        if 'vala' in langs:
-            if 'c' not in langs:
-                raise InterpreterException('Compiling Vala requires C. Add C to your project languages and rerun Meson.')
+        if 'vala' in langs and 'c' not in langs:
+            args.append('c')
 
         success = True
         for lang in sorted(args, key=compilers.sort_clink):

test cases/failing/93 vala without c/meson.build

@@ -1,2 +0,0 @@
-project('vala without c')
-add_languages('vala')

test cases/failing/93 vala without c/test.json

@@ -1,7 +0,0 @@
-{
-  "stdout": [
-    {
-      "line": "test cases/failing/93 vala without c/meson.build:2:0: ERROR: Compiling Vala requires C. Add C to your project languages and rerun Meson."
-    }
-  ]
-}