Remove inclusion of config.h fixes #124

Other libraries define the HAVE_CONFIG_H condition to include
"config.h", such as LibYAML or YAML.framework. When this happens
Mantle's extobjc tries to include its own "config.h" file which doesn't
exist. As this is not used in Mantle, so we can safely remove it.
Merge pull request #126 from Breek/have_config_h_removal

Remove inclusion of config.h fixes #124