Improved handling of corrupt databses and pkg-config support

• IMPORTANT: This release includes a number of important security fixes. Among these fixes is improved validation of the database metadata. Unfortunately, MaxMind GeoIP2 and GeoLite2 databases created earlier than January 28, 2014 had an invalid data type for the record_size in the metadata. Previously these databases worked on little endian machines with libmaxminddb but did not work on big endian machines. Due to increased safety checks when reading the file, these databases will no longer work on any platform. If you are using one of these databases, we recommend that you upgrade to the latest GeoLite2 or GeoIP2 database
• Added pkg-config support. If your system supports it, then running make install now installs a libmaxminddb.pc file for pkgconfig. Implemented by
  Jan Vcelak.
• Several segmentation faults found with afl-fuzz were fixed. These were
  caused by missing bounds checking and missing verification of data type.
• MMDB_get_entry_data_list will now fail on data structures with a depth
  greater than 512 and data structures that are cyclic. This should not
  affect any known MaxMind DB in production. All databases produced by
  MaxMind have a depth of less than five.