Add generated C file to mixed sources Vala test

This tests that generated C files make it past the filtering in
generate_vale_compile().

test cases/vala/11 mixed sources/c/foo.c

@@ -0,0 +1,5 @@
+int retval (void);
+
+int test (void) {
+    return retval ();
+}

test cases/vala/11 mixed sources/c/meson.build

@@ -0,0 +1,5 @@
+writec = find_program('writec.py')
+
+retval = custom_target('writec',
+  output : 'retval.c',
+  command : [writec, '@OUTPUT@'])

test cases/vala/11 mixed sources/c/writec.py

@@ -0,0 +1,12 @@
+#!/usr/bin/env python3
+
+import sys
+
+c = '''int
+retval(void) {
+  return 0;
+}
+'''
+
+with open(sys.argv[1], 'w') as f:
+    f.write(c)

test cases/vala/11 mixed sources/foo.c

@@ -1,3 +0,0 @@
-int test () {
-    return 0;
-}

test cases/vala/11 mixed sources/meson.build

@@ -2,4 +2,6 @@ project('foo', 'c', 'vala')
 
 glib = dependency('glib-2.0')
 
-executable('foo', 'foo.c', 'bar.vala', dependencies: [glib])
+subdir('c')
+e = executable('foo', 'c/foo.c', retval, 'vala/bar.vala', dependencies: [glib])
+test('test foo', e)