Allow searching LIBRARY_PATH with build_library=no

Instead of assuming the library is already present in bin/ when asking
not to build it, allow the linker to search the system for it as well.

Author: vilhalmer

tools/godotcpp.py

@@ -574,6 +574,12 @@ def _godot_cpp(env):
             default_args += ["compiledb"]
 
         env.Default(*default_args)
+    else:
+        # When not building the library, allow it to be found in other library
+        # paths on the system. It may have been built previously, so still add
+        # bin/ to the search path.
+        env.AppendUnique(LIBPATH=[env.Dir("bin/")])
+        env.AppendUnique(LIBS=[library_name])
 
     env.AppendUnique(LIBS=[env.File("bin/%s" % library_name)])
     return library