Update the top level CMakeLists.txt a bit (#1642)

Disable -Wunused-parameter in the top level CMakeLists.txt as this repo
has tons of it. Note: We have this particular warning disabled in
build-scripts/runtime_lib.cmake too.

Bump minimum cmake version for the top level CMakeLists.txt to fix the
following warning on macOS.

```
CMake Warning (dev):
  Policy CMP0042 is not set: MACOSX_RPATH is enabled by default. Run "cmake
  --help-policy CMP0042" for policy details. Use the cmake_policy command to
  set the policy and suppress this warning.
MACOSX_RPATH is not specified for the following targets: iwasm_shared
This warning is for project developers.  Use -Wno-dev to suppress it.
```

CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (C) 2019 Intel Corporation.  All rights reserved.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-cmake_minimum_required (VERSION 2.9)
+cmake_minimum_required (VERSION 3.0)
 
 project (iwasm)
 
@@ -107,7 +107,7 @@ set (WAMR_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 
 include (${WAMR_ROOT_DIR}/build-scripts/runtime_lib.cmake)
 
-set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wformat -Wformat-security -Wshadow")
+set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wformat -Wformat-security -Wshadow -Wno-unused-parameter")
 # set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wconversion -Wsign-conversion")
 
 set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wformat -Wformat-security -Wno-unused")