mysql_config:
- add not only $pkgincludedir, but also $pkgincludedir/.. to the header search path,
  for #include <mysql/plugin.h> to work


scripts/mysql_config.sh:
  - don't support headers in */include anymore. only in */include/mysql
  - remove the incorrect "bug fix" (fixed correctly long time ago)
  - add not only $pkgincludedir, but also $pkgincludedir/.. to the header search path,
    for #include <mysql/plugin.h> to work
  - but don't do it, if $pkgincludedir/.. is /usr/include