Undefined symbols for architecture x86_64:
In MacOS,

I was doing cpp program in eclipse and running this simple program 

```
#include<iostream>
#include<algorithm>
using namespace std;

int main(){
    int a,b,c,d,e,f=6;
    cin>>a>>b;

    c=max(a,b);
    d=6-c+1;

    e=__gcd(d,f);
    if (e==0){
        e=1;
    }
    d=d/e;
    f=f/e;

    cout<<d<<"/"<<f;
    return 0;
}
```

But I got this error: 

```
Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [codeforces] Error 1
"make all" terminated with exit code 2. Build might be incomplete.
```
-- codeforces was the name of my project 
And error was coming under using namespace std that 

> Symbol 'std' could not be resolved

And finally that binary not found

My Basic info is : 
```
~ % c++ -v

Apple clang version 12.0.5 (clang-1205.0.22.9)
Target: x86_64-apple-darwin20.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

~ % g++ -v                    

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1

Apple clang version 12.0.5 (clang-1205.0.22.9)
Target: x86_64-apple-darwin20.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

 ~ % make -v                   

GNU Make 3.81
Copyright (C) 2006  Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

This program built for i386-apple-darwin11.3.0
```
I saw one of the fixes that we can use `make clean_all && make -j4  `
But then I got this message: 

>  make: *** No rule to make target `clean_all'.  Stop.

Any kind of suggestions would be appreciated and I'll be grateful if you can suggest a good IDE for cpp in Mac (plz don't suggest Xcode as it is quite large).
Thanks in advance.
