Add an "int themable" field on struct t_config_option. The flag is set
automatically for every CONFIG_OPTION_TYPE_COLOR option, and may be set
explicitly on any other type by suffixing the type argument with
"|themable" in the call to config_file_new_option (e.g. "string|themable"
for a string option whose value contains "${color:...}" references).
Opt in the relevant string options in core and in the buflist, fset, irc,
relay plugins.
The flag is exposed via hdata, infolist, and print_log so scripts and
/debug can read it. This is the foundation for an upcoming /theme
command that will only be allowed to modify themable options.
At the moment, building WeeChat triggers several thousand -Wstrict-prototypes
diagnostics. This is due to its source code using an empty argument list for
functions and function pointers that take no arguments, instead of explicitly
declaring that they take no arguments by using a void list.
This commit replaces all empty argument lists with a void list.
Note that Ruby's headers also suffer the same problem, which WeeChat can't
do anything to fix. Thus, building WeeChat with the Ruby plugin enabled
will still issue approximately 30 such diagnostics.