Fix OSGPlugins_LIB_DIR on the MacOS build

Moves auto-detection of OSGPlugins_LIB_DIR into FindOSGPlugins.cmake.

This should hopefully fix the error in the MacOS build.

CMakeLists.txt

@@ -316,14 +316,6 @@ endif()
 if(OPENMW_USE_SYSTEM_OSG)
     find_package(OpenSceneGraph 3.3.4 REQUIRED ${USED_OSG_COMPONENTS})
     if(OSG_STATIC)
-        unset(OSGPlugins_LIB_DIR)
-        foreach(OSGDB_LIB ${OSGDB_LIBRARY})
-            # Skip library type names
-            if(EXISTS ${OSGDB_LIB} AND NOT IS_DIRECTORY ${OSGDB_LIB})
-                get_filename_component(OSG_LIB_DIR ${OSGDB_LIB} DIRECTORY)
-                list(APPEND OSGPlugins_LIB_DIR "${OSG_LIB_DIR}/osgPlugins-${OPENSCENEGRAPH_VERSION}")
-            endif()
-        endforeach(OSGDB_LIB)
         find_package(OSGPlugins REQUIRED COMPONENTS ${USED_OSG_PLUGINS})
     endif()
 endif()

cmake/FindOSGPlugins.cmake

@@ -15,6 +15,17 @@
 include(LibFindMacros)
 include(Findosg_functions)
 
+if (NOT OSGPlugins_LIB_DIR)
+    unset(OSGPlugins_LIB_DIR)
+    foreach(OSGDB_LIB ${OSGDB_LIBRARY})
+        # Skip library type names
+        if(EXISTS ${OSGDB_LIB} AND NOT IS_DIRECTORY ${OSGDB_LIB})
+            get_filename_component(OSG_LIB_DIR ${OSGDB_LIB} DIRECTORY)
+            list(APPEND OSGPlugins_LIB_DIR "${OSG_LIB_DIR}/osgPlugins-${OPENSCENEGRAPH_VERSION}")
+        endif()
+    endforeach(OSGDB_LIB)
+endif()
+
 if (NOT OSGPlugins_LIB_DIR)
     set(_mode WARNING)
     if (OSGPlugins_FIND_REQUIRED)