Look for MSVC 3rdparty deps differently.

Check for the 3rdparty dir in the parent of build dir, not the parent of the source dir. For the recommended build layout, this is the same location, but for super-builds using fgmeta it's not (and the source tree should not be touched).

If this causes anyone issues, please let me know, since it's possible more flexibility is needed to set the path explicitly.

CMakeLists.txt

@@ -114,7 +114,7 @@ option(ENABLE_TESTS     "Set to OFF to disable building SimGear's test applicati
 option(ENABLE_SOUND     "Set to OFF to disable building SimGear's sound support" ON)
 
 if (MSVC)
-  GET_FILENAME_COMPONENT(PARENT_DIR ${PROJECT_SOURCE_DIR} PATH)
+  GET_FILENAME_COMPONENT(PARENT_DIR ${PROJECT_BINARY_DIR} PATH)
   if (CMAKE_CL_64)
     SET(TEST_3RDPARTY_DIR "${PARENT_DIR}/3rdparty.x64")
   else (CMAKE_CL_64)