ruby: build dbm.so with our own gdbm_compat explicitly.

otherwise system libdb can be linked.

/reviewed-by @kirr
/reviewed-on nexedi/slapos!62

component/ruby/buildout.cfg

@@ -16,6 +16,7 @@ parts = ruby
 recipe = slapos.recipe.cmmi
 configure-options =
   --enable-shared --enable-rpath --disable-install-doc
+  --with-dbm-type=gdbm_compat
   --with-out-ext=tk,tk/tkutil,win32,win32ole
 environment =
   PATH=${xz-utils:location}/bin:%(PATH)s