mpers: add support of conditionally compiled printers

mpers: add support of conditionally compiled printers

We used to declare and define all printers marked with
MPERS_PRINTER_DECL, including ifdef'ed ones.  That approach left us
no way to conditionally compile mpersified printers, which was not
a problem until btrfs ioctls appeared on the horizon.

With this change, those mpersified printers that are not going
to be compiled are also won't be declared and won't be added to
struct_printers.

This is implemented by filtering all source files containing
MPERS_PRINTER_DECL markers through CPP.  As a nice side effect, this
also lifts an ugly requirement of writing all MPERS_PRINTER_DECL
declarations in a single line.

* README-mpers: Update description of MPERS_PRINTER_DECL syntax.
* defs.h [IN_MPERS_BOOTSTRAP] (MPERS_PRINTER_DECL): Turn into
a recursive variadic macro.
[!IN_MPERS_BOOTSTRAP] (MPERS_PRINTER_DECL): Turn into a variadic macro.
All callers changed.
* Makefile.am (mpers_preproc_files, mpers_printer_decl_pattern):
New variables.
(CLEANFILES): Add $(mpers_preproc_files).
(%.c.mpers.i): New rule.
(printers.h, %_printer_decls.h, %_printer_defs.h): Use
mpers_preproc_files instead of srcdir_mpers_source_files,
use mpers_printer_decl_pattern.
* .gitignore: Add /*.mpers.i.