Use mkmf to build C API specs
Use mkmf to build C API specs

* Avoids duplicating a lot of logic from mkmf.
* Unify with compile_truffleruby_extconf_make().

ruby/spec,ruby/rubyspec,eregon/rubyspec,ruby/spec,nobu/rubyspec,sgarciac/spec,nobu/rubyspec,sgarciac/spec,ruby/spec,nobu/rubyspec,ruby/rubyspec,sgarciac/spec,eregon/rubyspec,eregon/rubyspec