build: use CMAKE_POSITION_INDEPENDENT_CODE instead of -fPIC (#21947)

It's simpler to let cmake figure out what flags to add to each platforms
to create position independent code rather than handling it ourselves.

Also remove code that sets POSITION_INDEPENDENT_CODE property on SHARED
and MODULE libraries, as it's already on by default.

cmake.deps/cmake/BuildMsgpack.cmake

@@ -5,13 +5,9 @@ set(MSGPACK_CMAKE_ARGS
     -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
     -DCMAKE_GENERATOR=${CMAKE_GENERATOR}
     -DCMAKE_GENERATOR_PLATFORM=${CMAKE_GENERATOR_PLATFORM}
+    -DCMAKE_POSITION_INDEPENDENT_CODE=ON
     ${BUILD_TYPE_STRING})
 
-if(NOT MSVC)
-  list(APPEND MSGPACK_CMAKE_ARGS
-    "-DCMAKE_C_FLAGS:STRING=-fPIC")
-endif()
-
 if(USE_EXISTING_SRC_DIR)
   unset(MSGPACK_URL)
 endif()