Support for MSVC compiler: Ensure mode_t gets defined. * m4/fcntl_h.m4 (gl_FCNTL_H): Require AC_TYPE_MODE_T. * m4/spawn_h.m4 (gl_SPAWN_H): Likewise. * m4/sys_stat_h.m4 (gl_HEADER_SYS_STAT_H): Likewise. * m4/sys_types_h.m4 (gl_SYS_TYPES_H): Likewise. * tests/test-fcntl-h.c: Check that mode_t is defined. * tests/test-sys_stat.c: Likewise. * tests/test-sys_types.c: Likewise. * doc/posix-headers/fcntl.texi: Mention the missing mode_t problem. * doc/posix-headers/sys_stat.texi: Likewise. * doc/posix-headers/sys_types.texi: Likewise.