add _HEADERS primaries to Makefile.am snippets for installable headers * modules/alloca-opt, modules/arg-nonnull, modules/argz, modules/arpa_inet, modules/byteswap, modules/c++defs, modules/ctype, modules/dirent, modules/errno, modules/fcntl-h, modules/float, modules/fnmatch, modules/getopt-posix, modules/glob, modules/iconv-h, modules/iconv_open, modules/inttypes, modules/langinfo, modules/link-warning, modules/locale, modules/math, modules/netdb, modules/netinet_in, modules/poll-h, modules/pthread, modules/pty, modules/sched, modules/search, modules/selinux-h, modules/signal, modules/spawn, modules/stdarg, modules/stdbool, modules/stddef, modules/stdint, modules/stdio, modules/stdlib, modules/string, modules/strings, modules/sys_file, modules/sys_ioctl, modules/sys_select, modules/sys_socket, modules/sys_stat, modules/sys_time, modules/sys_times, modules/sys_utsname, modules/sys_wait, modules/sysexits, modules/termios, modules/time, modules/unistd, modules/unitypes, modules/unused-parameter, modules/warn-on-use, modules/wchar, modules/wctype (Makefile.am): Add nodist_include_HEADERS, nobase_nodist_include_HEADERS and EXTRA_HEADERS declarations for potentially installable header files. * gnulib-tool: Initialize nodist_include_HEADERS, nobase_nodist_include_HEADERS and EXTRA_HEADERS to prime for new `.*_HEADERS +=' declarations. (func_emit_lib_Makefile_am): Edit away new nodist_include_HEADERS, nobase_nodist_include_HEADERS and EXTRA_HEADERS declarations by default, reverting the Makefile.am snippets above to their original condition prior to this change *unless* the gnulib library is being installed.