Prohibit absolute paths in subdir().

mesonbuild/interpreter.py

@@ -2132,6 +2132,8 @@ requirements use the version keyword argument instead.''')
             raise InvalidArguments('Must not go into subprojects dir with subdir(), use subproject() instead.')
         prev_subdir = self.subdir
         subdir = os.path.join(prev_subdir, args[0])
+        if os.path.isabs(subdir):
+            raise InvalidArguments('Subdir argument must be a relative path.')
         if subdir in self.visited_subdirs:
             raise InvalidArguments('Tried to enter directory "%s", which has already been visited.'
                                    % subdir)

test cases/failing/42 abs subdir/bob/meson.build

@@ -0,0 +1,2 @@
+# This file is never reached.
+x = 3

test cases/failing/42 abs subdir/meson.build

@@ -0,0 +1,6 @@
+project('abs subdir', 'c')
+
+# For some reason people insist on doing this, probably
+# because Make has taught them to never rely on anything.
+subdir(join_paths(meson.source_root(), 'bob'))
+