🐛 warning: 'TARGET_OS_MAC' is not defined, evaluates to 0 [-Wundef-prefix=TARGET_OS_]

[AliYusuf95]

Fiber version
v2.20.1

Issue description
If I use monitor middleware and try to build my project on macOS using LLVM/Clang 11.0 or higher (in my case 12.0.0), I get the following error:

# github.com/gofiber/fiber/v2/internal/gopsutil/cpu
../../Go/pkg/mod/github.com/gofiber/fiber/v2@v2.20.1/internal/gopsutil/cpu/cpu_darwin_cgo.go:13:5: warning: 'TARGET_OS_MAC' is not defined, evaluates to 0 [-Wundef-prefix=TARGET_OS_]

Code snippet

package main

import (
	"log"
	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/monitor"
)

func main() {
	app := fiber.New()
	
	app.Get("/dashboard", monitor.New())
	
	log.Fatal(app.Listen(":3000"))
}

Proposed Solution
Based on this issue it seems adding #include <TargetConditionals.h> before using TARGET_OS_MAC solves the problem.
As a workaround calling CGO_CFLAGS=-Wno-undef-prefix go build remove the warning message.