building with system libmagic fails with no implicit conversion of nil into String (TypeError)
extconf.rb:301:in `+': no implicit conversion of nil into String (TypeError)
from extconf.rb:301:in `<main>'
ruby-magic-static 0.3.5 was successful with the same version of libmagic (this is when packaging this gem for debian).
export RB_MAGIC_USE_SYSTEM_LIBRARIES = true is set to force using system libmagic.