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.
Support variadic macros.
Merge pull request #137 from Typz/variadic_macros

Support variadic macros.
Merge pull request #138 from Typz/gcc_ternary

Extension: GCC's conditionals with omitted operands.
Support GCC's Conditionals with Omitted Operands #138
Mark the rule change as extension
Merge branch 'master' of https://github.com/wenns/sonar-cxx into feature/recovery-from-parsing-errors