Move Ruby-related checks from autoconf to extconf.rb; add extra check for rb_alloc_func_t

Ruby ships its own config.h which may or may not define the same
relevant constants as our autoconf-generated config.h. Instead of
trying to specify the exact path to the wanted header file we may just
as well simply use Ruby's autoconf-inspired checks and macros.

ruby/ext/guestfs/extconf.rb.in

@@ -29,6 +29,10 @@ unless have_library("guestfs", "guestfs_create", "guestfs.h")
   raise "libguestfs not found"
 end
 
+have_func("rb_hash_lookup")
+have_func("rb_define_alloc_func")
+have_type("rb_alloc_func_t")
+
 $CFLAGS =
   "#{$CFLAGS} @CFLAGS@ -DGUESTFS_PRIVATE=1 " <<
   "@WARN_CFLAGS@ @WERROR_CFLAGS@"