openvdb/Makefile: allow users to customize variables using a file

Teach the Makefile to optionally include "config.mak" so that users
can customize the build variables by providing a file.

Add "config.mak" to .gitignore so that users do not accidentally
commit their custom configuration.

Signed-off-by: David Aguilar <david.aguilar@disneyanimation.com>