Use cmake POSITION_INDEPENDENT_CODE instead of hardcoding -pie -fPIE (#1598)

This fixes unused option warnings on -pie for macOS.
(On macOS cmake produces "-fPIE -Xlinker -pie")

Bump required cmake version to 3.14 for CheckPIESupported.

References:
https://cmake.org/cmake/help/latest/prop_tgt/POSITION_INDEPENDENT_CODE.html
https://cmake.org/cmake/help/latest/module/CheckPIESupported.html#module:CheckPIESupported

samples/ref-types/CMakeLists.txt

@@ -1,7 +1,9 @@
 # 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 3.14)
+
+include(CheckPIESupported)
 
 if (NOT WAMR_BUILD_PLATFORM STREQUAL "windows")
   project(ref-types)
@@ -70,7 +72,6 @@ set(WAMR_BUILD_REF_TYPES 1)
 
 if (NOT MSVC)
   # compiling and linking flags
-  set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -pie -fPIE")
   if (NOT (CMAKE_C_COMPILER MATCHES ".*clang.*" OR CMAKE_C_COMPILER_ID MATCHES ".*Clang"))
     set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,--gc-sections")
   endif ()
@@ -107,6 +108,9 @@ include (${SHARED_DIR}/utils/uncommon/shared_uncommon.cmake)
 
 add_executable(hello src/hello.c ${UNCOMMON_SHARED_SOURCE})
 
+check_pie_supported()
+set_target_properties (hello PROPERTIES POSITION_INDEPENDENT_CODE ON)
+
 target_include_directories(hello PRIVATE ${UNCOMMON_SHARED_DIR})
 
 target_link_libraries(hello vmlib -lpthread -lm)