The -ftrapv doesn't seem to want to generate code that will trap on overflow with optimization.  I've tried with and without -fwrapv to try and get it to work.

$ cat t.c
#include <stdio.h>
int foo( int a) {
    if (a + 100 < a) {
        printf("Integer overflow detected!\n");
    } else {
        printf("Integer overflow not detected!\n");
    }
    return a;
}

int main() {
    foo(0x7fffffff);
}
mrs2 $ ./xgcc -B./ -fno-wrapv -ftrapv t.c -O2
mrs2 $ a.out
Integer overflow detected!
mrs2 $ ./xgcc -B./ -fwrapv -ftrapv t.c -O2
mrs2 $ a.out
Integer overflow detected!
mrs2 $ ./xgcc -B./ -fwrapv -ftrapv t.c    
mrs2 $ a.out
Abort trap
mrs2 $ ./xgcc -B./ -fno-wrapv -ftrapv t.c    
mrs2 $ a.out
Abort trap