Make generated files be read-only.

Make generated files be read-only.
* modules/alloca-opt (Makefile.am): Work also when $@ is read-only.
Ensure that each generated file is now read-only.
* modules/argz: Likewise.
* modules/arpa_inet: Likewise.
* modules/byteswap: Likewise.
* modules/configmake: Likewise.
* modules/fcntl: Likewise.
* modules/fnmatch: Likewise.
* modules/getopt: Likewise.
* modules/glob: Likewise.
* modules/inttypes: Likewise.
* modules/netinet_in: Likewise.
* modules/poll: Likewise.
* modules/stdbool: Likewise.
* modules/stdint: Likewise.
* modules/sys_select: Likewise.
* modules/sys_socket: Likewise.
* modules/sys_stat: Likewise.
* modules/sysexits: Likewise.
* modules/localcharset: Same as above, but continue using temporary
file named "t-$@" (why different?) rather than the "$@-t" used
everywhere else.

* modules/sysexits (Makefile.am): Replace literal occurrences
of "sysexit.h" more readable, and more consistent, "$@".