CMake: Fix support for OSG static lib paths

`$<TARGET_FILE:...>` fails if the argument is already a path (instead of
a library name). This happens when a static library is found via
`osg_find_library`.

If the argument contains `/` or `.`, do not use `$<TARGET_FILE:...>`

apps/opencs/CMakeLists.txt

@@ -233,7 +233,11 @@ if(OSG_STATIC)
     add_library(openmw_cs_osg_plugins INTERFACE)
     foreach(_plugin ${USED_OSG_PLUGINS})
         string(TOUPPER ${_plugin} _plugin_uc)
-        list(APPEND _osg_plugins_static_files $<TARGET_FILE:${${_plugin_uc}_LIBRARY}>)
+        if (${_plugin_uc}_LIBRARY MATCHES "[/.]")
+            list(APPEND _osg_plugins_static_files ${${_plugin_uc}_LIBRARY})
+        else()
+            list(APPEND _osg_plugins_static_files $<TARGET_FILE:${${_plugin_uc}_LIBRARY}>)
+        endif()
         target_link_libraries(openmw_cs_osg_plugins INTERFACE ${${_plugin_uc}_LIBRARY})
     endforeach()
     # We use --whole-archive because OSG plugins use registration.

apps/openmw/CMakeLists.txt

@@ -148,7 +148,11 @@ if(OSG_STATIC)
     add_library(openmw_osg_plugins INTERFACE)
     foreach(_plugin ${USED_OSG_PLUGINS})
         string(TOUPPER ${_plugin} _plugin_uc)
-        list(APPEND _osg_plugins_static_files $<TARGET_FILE:${${_plugin_uc}_LIBRARY}>)
+        if (${_plugin_uc}_LIBRARY MATCHES "[/.]")
+            list(APPEND _osg_plugins_static_files ${${_plugin_uc}_LIBRARY})
+        else()
+            list(APPEND _osg_plugins_static_files $<TARGET_FILE:${${_plugin_uc}_LIBRARY}>)
+        endif()
         target_link_libraries(openmw_osg_plugins INTERFACE ${${_plugin_uc}_LIBRARY})
     endforeach()
     # We use --whole-archive because OSG plugins use registration.