I'm not the language you're looking for!
C89/C99, 171 152 136 114 111 107 105 bytes

Thanks at @Hurkyls, @Qwertiys, @jimmy23013 and @MD XF for your hints.

golfed version:

c;main(){c=-4.5//**/
-4.5;printf("This program wasn't written in C%d, it was built for C%d!",90-c,98+c);}


ungolfed version:

c;

main()
{
    c = -4.5//**/
    -4.5;
    printf("This program wasn't written in C%d, it was built for C%d!",90-c,98+c);
}


Little description:

C versions previous C99 just had the multiline comment like this:

/*foo*/


with C99 the single line comment was introduced. like this:

//foo


so if you compile a line like this:

c =-4.5//**/
-4.5;


the for the c99 compiler compiling-related code would be:

c = -4.5 -4.5;


while the for a c89 compiler relevant code would be:

(as the first / isn't part of a comment and therfor treat as operator)

c = -4.5 / -4.5;