Guard against subdir segments in output files.

mesonbuild/interpreter.py

@@ -1816,6 +1816,8 @@ class Interpreter():
             raise InterpreterException('Input must be a string.')
         if not isinstance(output, str):
             raise InterpreterException('Output must be a string.')
+        if os.path.split(output)[0] != '':
+            raise InterpreterException('Output file name must not contain a subdirectory.')
         if 'configuration' in kwargs:
             conf = kwargs['configuration']
             if not isinstance(conf, ConfigurationDataHolder):

test cases/failing/27 output subdir/foo.in

@@ -0,0 +1 @@
+Nothing here.

test cases/failing/27 output subdir/meson.build

@@ -0,0 +1,5 @@
+project('outdir path', 'c')
+
+configure_file(input : 'foo.in',
+  output : 'subdir/foo',
+  configuration : configuration_data())

test cases/failing/27 output subdir/subdir/dummy.txt

@@ -0,0 +1,2 @@
+I'm only here because Git is stupid about empty dirs.
+