Make it a yes/no/auto combo. "yes" means all modules are built into libgtk,
"no" that none are and "auto" uses the platform defaults, yes on win32,
no otherwise.
If we need more we can always extend it later.
This changes the configure option into two states:
auto: build all that can be build (default)
A list of backend names: build them and fail if we can't
"papi" is missing because it's not in Debian and I can't test it.
Build the input modules for GTK+, either as modules or built directly
into GTK. Also provide a configure option to build the specified
immodules, or all, or the backend immodule(s) or none of the immodules
into GTK. Note that for Visual Studio all immodules are built into
the GTK DLL by default, like what is done in the Visual Studio projects.
Note that building the backend immodules for Quartz, X11 and Wayland are
currently untested.
