Allow spl_config.h to be included by dependant packages (updated)

We need dependent packages to be able to include spl_config.h to
build properly.  This was partially solved in commit 0cbaeb1 by using
AH_BOTTOM to #undef common #defines (PACKAGE, VERSION, etc) which
autoconf always adds and cannot be easily removed.  This solution
works as long as the spl_config.h is included before your projects
config.h.  That turns out to be easier said than done.  In particular,
this is a problem when your package includes its config.h using the
-include gcc option which ensures the first thing included is your
config.h.

To handle all cases cleanly I have removed the AH_BOTTOM hack and
replaced it with an AC_CONFIG_HEADERS command.  This command runs
immediately after spl_config.h is written and with a little awk-foo
it strips the offending #defines from the file.  This eliminates
the problem entirely and makes header safe for inclusion.

Also in this change I have removed the few places in the code where
spl_config.h is included.  It is now added to the gcc compile line
to ensure the config results are always available.

Finally, I have also disabled the verbose kernel builds.  If you
want them back you can always build with 'make V=1'.  Since things
are working now they don't need to be on by default.

cmd/Makefile.in

@@ -198,7 +198,8 @@ target_alias = @target_alias@
 target_cpu = @target_cpu@
 target_os = @target_os@
 target_vendor = @target_vendor@
-DEFAULT_INCLUDES = -I${top_srcdir} -I${top_srcdir}/lib
+DEFAULT_INCLUDES = -include ${top_srcdir}/spl_config.h \
+	-I${top_srcdir}/lib
 AM_CFLAGS = -Wall -Wstrict-prototypes -Werror -Wshadow \
 	-D__USE_LARGEFILE64
 spl_SOURCES = spl.c