[patch] Check for plugin installation directory in meshlab_tools.cmake.

[shamefulCake1]

This is a simple patch to avoid a cryptic "missing install target" message from cmake.

I cannot submit a pull request, because of the broken Github 2FA.

diff --git a/src/cmake/meshlab_tools.cmake b/src/cmake/meshlab_tools.cmake
index 4df1f2d4d..14df335df 100644
--- a/src/cmake/meshlab_tools.cmake
+++ b/src/cmake/meshlab_tools.cmake
@@ -4,6 +4,9 @@
 
 #function used to add plugins
 function(add_meshlab_plugin target_name)
+	if(NOT DEFINED MESHLAB_PLUGIN_INSTALL_DIR)
+		message(FATAL_ERROR "MESHLAB_PLUGIN_INSTALL_DIR not defined, check your cmake usage.")
+	endif()
 	set(EXCLUDE_ALL_ARG "")
 	if (MESHLAB_BUILD_MINI)
 		set(EXCLUDE_ALL_ARG "EXCLUDE_FROM_ALL")