MariaDB-devel is incomplete for plugins

MariaDB-devel rpm package (and the corresponding deb package) are supposed to allow plugins to be built without a server. It mostly works, but there a few missing bits that need to be fixed:

• HAVE_WSREP is defined to 1 in my_global.h while it's what the server uses, it makes server headers not usable for plugins as they start to pull in (missing) wsrep headers.
  • Suggestion:
    • remove it from the header
    • use -DWITH_WSREP=1 on the command line
• plugin.cmake is not installed, so plugins cannot be built. Also, plugin.cmake needs install_layout.cmake and has a couple of dtrace/windows macros.
  • Suggestion:
    • combine install_layout.cmake and plugin.cmake into one file, say, mariadb-plugin-config.cmake
    • include it into MariaDB-devel
    • define a dummy dtrace macro there, unless it's already defined
    • Not sure what to do with ADD_VERSION_INFO().
    • CONFIGURE_PLUGINS() should likely not be in this new mariadb-plugin-config.cmake, it'll stay in the server
    • create an alias MARIADB_ADD_PLUGIN, because it's time
    • perhaps mariadb-plugin-config.cmake should also do

      INCLUDE_DIRECTORIES(/usr/include/mysql/server /usr/include/mysql/server/private)

      this is an argument that mariadb-plugin-config.cmake should be generated.

This way a plugin can have a top-level CMakeLists.txt with

FIND_PACKAGE(mariadb-plugin REQUIRED)

MARIADB_ADD_PLUGIN(myplugin SOURCE my_plugin.c)

and it'll just work.