Remove ONLY_IF clause in MYSQL_ADD_PLUGIN and the requirement
that every plugin's CMakeLists.txt *must* do MYSQL_ADD_PLUGIN
for PLUGIN_XXX=YES to work. This was very fragile and cannot be
relied on.

Use a different implementation of =YES check - iterate all
PLUGIN_* variables and see which one doesn't have a matching target.

Revert all ONLY_IF changes in CMakeLists.txt files.