comparison between signed and unsigned integer expressions and 0x80000000

I have the following code:

#include <iostream>

using namespace std;

int main()
{
    int a = 0x80000000;
    if(a == 0x80000000)
        a = 42;
    cout << "Hello World! :: " << a << endl;
    return 0;
}

The output is

Hello World! :: 42

so the comparison works. But the compiler tells me

g++ -c -pipe -g -Wall -W -fPIE  -I../untitled -I. -I../bin/Qt/5.4/gcc_64/mkspecs/linux-g++ -o main.o ../untitled/main.cpp
../untitled/main.cpp: In function 'int main()':
../untitled/main.cpp:8:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if(a == 0x80000000)
             ^

So the question is: Why is 0x80000000 an unsigned int? Can I make it signed somehow to get rid of the warning?

As far as I understand, 0x80000000 would be INT_MIN as it's out of range for positive a integer. but why is the compiler assuming, that I want a positive number?

I'm compiling with gcc version 4.8.1 20130909 on linux.