This wasn't an easy riddle by any means. One has to either have a lot of experience developing both C and C++, or take a very hard look at the standards to spot the differences. Bottom line? I couldn't find a solution. Fortunately a brother and a colleague both came through:
Solution 1: Ugly Hack
Tal (one of my colleagues at Monfort) came up with the following concept:
int a = 1//**/10
printf( "%d\n", a );
This deserves some explanation: originally C did not support single-line comments (//); these were added to a later version of the standard called C99. So what happens is, if you compile this code with an old C compiler the "//**/" string is parsed as a division operator and an empty comment, resulting in 1/10=0. A modern C compiler (or any C++ compiler) will treat the entire line as a comment and the result is 1. One way to easily test this is to compile with gcc -std=c89 file.c.
Solution 2: Elegant Hack
I didn't have the time to properly look for a detailed comparison of C and C++ standards (I'll save you the time: this looks to be a good source). There are probably quite a few ways to achieve the goal, however my brother Mickey suggested one I find quite elegant:
printf( "%d\n", sizeof( 'c' ) == sizeof( char ) );
Seems odd? Apparently one of the (relatively small) breaking changes between C and C++ is how character literals are handled. In C a character literal takes the type int, whereas in C++ a character literal has the expected size (source).