krita: Add a proper option for enable/disable GMic

I can never remember what variable to set to disable GMic, so instead
use an option so that ccmake/cmake-gui can at least list it.

krita/plugins/extensions/CMakeLists.txt

@@ -14,11 +14,9 @@ add_subdirectory( separate_channels )
 add_subdirectory( shearimage )
 add_subdirectory( dockers )
 add_subdirectory( layergroupswitcher )
+
 # Allow to skip building GMIC plugin
-if(NOT DEFINED WITH_GMIC)
+option(WITH_GMIC "Build the G'Mic plugin" ON)
+if(WITH_GMIC)
     add_subdirectory( gmic )
-else(NOT DEFINED WITH_GMIC)
-    if(WITH_GMIC)
-        add_subdirectory( gmic )
-    endif(WITH_GMIC)
-endif(NOT DEFINED WITH_GMIC)
+endif(WITH_GMIC)