gnulib-tool: Better isolation between different gnulib-tool invocations.

gnulib-tool: Better isolation between different gnulib-tool invocations.

* gnulib-tool: New option --witness-c-macro.
(witness_c_macro): New variable.
(func_emit_lib_Makefile_am): If --witness-c-macro was specified, let
AM_CPPFLAGS define it as a C macro.
(func_emit_tests_Makefile_am): Likewise.
(func_import): Store witness_c_macro setting in gnulib-cache.m4 and
read it from there.
* m4/gnulib-common.m4 (gl_MODULE_INDICATOR_CONDITION): Define through
m4_define, not AC_DEFUN.
(gl_MODULE_INDICATOR_SET_VARIABLE_AUX,
gl_MODULE_INDICATOR_SET_VARIABLE_AUX_OR): New macros.
(gl_MODULE_INDICATOR_SET_VARIABLE): Use them.
* modules/arpa_inet (Makefile.am): Use sed expression s/.../.../, not
s|...|...|, to substitute the values of the GNULIB_* module indicator
variables.
* modules/dirent (Makefile.am): Likewise.
* modules/fcntl-h (Makefile.am): Likewise.
* modules/iconv-h (Makefile.am): Likewise.
* modules/langinfo (Makefile.am): Likewise.
* modules/locale (Makefile.am): Likewise.
* modules/math (Makefile.am): Likewise.
* modules/netdb (Makefile.am): Likewise.
* modules/poll-h (Makefile.am): Likewise.
* modules/pty (Makefile.am): Likewise.
* modules/search (Makefile.am): Likewise.
* modules/signal (Makefile.am): Likewise.
* modules/spawn (Makefile.am): Likewise.
* modules/stdio (Makefile.am): Likewise.
* modules/stdlib (Makefile.am): Likewise.
* modules/string (Makefile.am): Likewise.
* modules/sys_ioctl (Makefile.am): Likewise.
* modules/sys_select (Makefile.am): Likewise.
* modules/sys_socket (Makefile.am): Likewise.
* modules/sys_stat (Makefile.am): Likewise.
* modules/sys_times (Makefile.am): Likewise.
* modules/sys_utsname (Makefile.am): Likewise.
* modules/sys_wait (Makefile.am): Likewise.
* modules/termios (Makefile.am): Likewise.
* modules/time (Makefile.am): Likewise.
* modules/unistd (Makefile.am): Likewise.
* modules/wchar (Makefile.am): Likewise.