Should gnulib lib_SOURCES contain .h files? also, splitting up ftoastr

[Paul Eggert]

Emacs needs the dtoastr function, but not ftoastr or ldtoastr.
To save build time, it'd be nice if it could build just dtoastr.
The easiest way I can think of to do that is to split up the
existing ftoastr module (which supports all three functions)
into three modules: ftoastr, dtoastr, ldtoastr, one for
each function.  (Perhaps there's a better way? but I can't
think of it offhand.)

While looking into this I noticed that ftoastr, like some
but not all other modules, puts a .h file into lib_SOURCES:

lib_SOURCES += ftoastr.h ftoastr.c dtoastr.c ldtoastr.c

Now I naively had thought that lib_SOURCES was just for
.c files that produce .o files.  Is my assumption incorrect?
I see that other modules are inconsistent in this area.
If I'm right, other modules should be altered so that lib_SOURCES
does not mention .h files; if I'm wrong, I'd like to know
what lib_SOURCES is for, and perhaps other modules need to
be altered so that their lib_SOURCES mention .h files.