Force C standard to gnu11, otherwise more recent GCC version will
result in a number of different prototypes (compared to koxtoolchain),
e.g.:

```diff
--- i/ffi/mupdf_h.lua
+++ w/ffi/mupdf_h.lua
@@ -308,8 +308,8 @@ typedef struct {
   int do_labels;
 } pdf_write_options;
 void *mupdf_pdf_save_document(fz_context *, pdf_document *, const char *, pdf_write_options *);
-fz_alloc_context *mupdf_get_my_alloc_context();
-int mupdf_get_cache_size();
+fz_alloc_context *mupdf_get_my_alloc_context(void);
+int mupdf_get_cache_size(void);
 int mupdf_error_code(fz_context *);
 char *mupdf_error_message(fz_context *);
 fz_matrix *mupdf_fz_scale(fz_matrix *, float, float);
```