Add WITH_GRASS_PLUGIN cmake setting to determine whether the GRASS

c++ plugin is built

Allows building the GRASS provider without the plugin

src/plugins/CMakeLists.txt

@@ -14,7 +14,10 @@ if (WITH_SPATIALITE)
 endif()
 
 if (GRASS_FOUND)
-  add_subdirectory(grass)
+  set (WITH_GRASS_PLUGIN TRUE CACHE BOOL "Determines whether the GRASS c++ plugin should be built")
+  if (WITH_GRASS_PLUGIN)
+    add_subdirectory(grass)
+  endif()
 endif()
 
 if (CUSTOM_PLUGINS)