* m4/truncate.m4 (gl_FUNC_TRUNCATE): Test also whether 'truncate' is
declared. Set HAVE_DECL_TRUNCATE, not HAVE_TRUNCATE.
* lib/unistd.in.h (truncate): Test HAVE_DECL_TRUNCATE, not
HAVE_TRUNCATE.
* modules/truncate: Likewise.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize HAVE_DECL_TRUNCATE,
not HAVE_TRUNCATE.
* modules/unistd (Makefile.am): Substitute HAVE_DECL_TRUNCATE, not
HAVE_TRUNCATE.
* doc/posix-functions/truncate.texi: Mention the issue.