fopen: Silence a gcc warning

[Bruno Haible]

The continuous integration of 'gzip' failed today:

  CC       fopen.o
fopen.c: In function 'rpl_fopen':
fopen.c:50:7: error: variable 'open_direction' set but not used 
[-Werror=unused-but-set-variable]
   50 |   int open_direction;
      |       ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[6]: *** [Makefile:2311: fopen.o] Error 1

Contributing factors are:
  - There is a depencency from module 'fclose' to module 'fopen' since last
    week.
  - The variable 'open_direction' is set but possibly not used (depending
    on #if conditions).
  - gzip's configuration apparently enables many warnings and -Werror.
  - The fopen module uses AC_LIBOBJ, in order to avoid link error w.r.t.
    to modules fopen vs. fopen-gnu.
  - In AC_LIBOBJ compilation units, Gnulib's override of the CFLAGS with
    -Wno-error has no effect.

The easiest fix is the following:


2023-05-02  Bruno Haible  <bruno@clisp.org>

        fopen: Silence a gcc warning.
        * lib/fopen.c (rpl_fopen): Mark open_direction as used.

diff --git a/lib/fopen.c b/lib/fopen.c
index f8469a0bbc..e1e4cdbd23 100644
--- a/lib/fopen.c
+++ b/lib/fopen.c
@@ -225,5 +225,9 @@ rpl_fopen (const char *filename, const char *mode)
     }
 #endif
 
+  /* open_direction is sometimes used, sometimes unused.
+     Silence gcc's warning about this situation.  */
+  (void) open_direction;
+
   return orig_fopen (filename, mode);
 }