fixed broken build when user provides their own onnxruntime library (#201)

@@ -247,7 +247,11 @@ endif()
 message(STATUS "location_onnxruntime_header_dir: ${location_onnxruntime_header_dir}")
 
 if(DEFINED ENV{SHERPA_ONNXRUNTIME_LIB_DIR})
-  set(location_onnxruntime_lib $ENV{SHERPA_ONNXRUNTIME_LIB_DIR}/libonnxruntime.so)
+  if(APPLE)
+    set(location_onnxruntime_lib $ENV{SHERPA_ONNXRUNTIME_LIB_DIR}/libonnxruntime.dylib)
+  else()
+    set(location_onnxruntime_lib $ENV{SHERPA_ONNXRUNTIME_LIB_DIR}/libonnxruntime.so)
+  endif()
   if(NOT EXISTS ${location_onnxruntime_lib})
     set(location_onnxruntime_lib $ENV{SHERPA_ONNXRUNTIME_LIB_DIR}/libonnxruntime.a)
   endif()