fopen-gnu: Allow use as dependency from test modules.
* m4/fopen.m4 (gl_FUNC_FOPEN_GNU): Set REPLACE_FOPEN_FOR_FOPEN_GNU
instead of REPLACE_FOPEN.
* m4/stdio_h.m4 (gl_STDIO_H_REQUIRE_DEFAULTS): Initialize the fopen-gnu
module indicator.
(gl_STDIO_H_DEFAULTS): Initialize REPLACE_FOPEN_FOR_FOPEN_GNU.
* modules/stdio (Makefile.am): Substitute GNULIB_FOPEN_GNU,
REPLACE_FOPEN_FOR_FOPEN_GNU.
* modules/fopen-gnu (Depends-on): Add more dependencies.
(configure.ac): Test REPLACE_FOPEN_FOR_FOPEN_GNU instead of
REPLACE_FOPEN.
* lib/stdio.in.h (fopen): Test REPLACE_FOPEN,
REPLACE_FOPEN_FOR_FOPEN_GNU, and the respective module indicators
instead of just REPLACE_FOPEN.