Add test for usage of generated File sources in libraries

This tests the previous commit:

 vs: Fix usage of mesonlib.File.rel_to_builddir with generated files

test cases/common/27 library versions/meson.build

@@ -5,3 +5,5 @@ shared_library('somelib', 'lib.c',
   name_suffix : 'suffix',
   install_dir : 'lib',
   install : true)
+
+subdir('subdir')

test cases/common/27 library versions/subdir/meson.build

@@ -0,0 +1,8 @@
+# Test that using files generated with configure_file as sources works.
+# We do this inside a subdir so that the path isn't accidentally correct
+# because there is no structure in the build dir.
+genlib = configure_file(input : '../lib.c',
+  output : 'genlib.c',
+  configuration : configuration_data())
+shared_library('genlib', genlib,
+  install : false)