sys_types: Ensure off64_t is defined on all platforms. * m4/off64_t.m4: New file. * m4/sys_types_h.m4 (gl_SYS_TYPES_H): Require gl_TYPE_OFF64_T. * lib/sys_types.in.h (off64_t): New type. (GNULIB_defined_off64_t): New macro. * modules/sys_types (Files): Add m4/off64_t.m4. (configure.ac-early): Require AC_USE_SYSTEM_EXTENSIONS. (Makefile.am): Substitute HAVE_OFF64_T. * tests/test-sys_types.c: Verify that off64_t is defined. * doc/posix-headers/sys_types.texi: Mention the off64_t workaround.