remove #define __attribute__(A) from plugin.h
increase API version because placeholders were added
more robust definition of min_plugin_interface_version


include/mysql/plugin.h:
  move #define __attribute__(A) out of plugin.h (not part of the API)
  increase API version because placeholders were added
plugin/fulltext/plugin_example.c:
  compilation failure on non-GCC compilers
sql/sql_plugin.cc:
  more robust definition of min_plugin_interface_version
  it should work even when we forget to update it