Support GCC's Conditionals with Omitted Operands.

There is a GCC extension which allows the middle operand in a conditional
expression to be omitted. Therefore, the expression "x ? : y" has the value of x
if that is nonzero; otherwise, the value of y.
Merge pull request #138 from Typz/gcc_ternary

Extension: GCC's conditionals with omitted operands.