Output difference in GCC and Turbo C

Why is there a difference in the output produced when the code is compiled using the two compilers GCC and Turbo C.

#include <stdio.h>

int main()
{    
    char *p = "I am a string";
    char *q = "I am a string";

    if(p==q)
    {
        printf("Optimized");
    }
    else{
        printf("Change your compiler");
    }
    return 0;
}

I get Optimized on GCC and Change your compiler on Turbo C. Why?

Your questions has been tagged C as well as C++. So I'd answer for both the languages.

C

From ISO C99 (Section 6.4.5/6)

It is unspecified whether these arrays are distinct provided their elements have the appropriate values.

That means it is unspecified whether p and q are pointing to the same string literal or not. In case of gcc they both are pointing to "I am a string" (gcc optimizes your code) whereas in Turbo C they are not.

_{Unspecified Behavior: Use of an unspecified value, or other behavior where this International Standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance}

C++

From ISO C++-98 (Section 2.13.4/2)

Whether all string literals are distinct(that is, are stored in non overlapping objects) is implementation defined.

In C++ your code invokes Implementation defined behaviour.

_{Implementation-defined Behavior: Unspecified Behavior where each implementation documents how the choice is made}

Also see this question.