Improve Assert macro

- do not use anymore gcc extension for __VA_ARGS__

- check that parameters are always correct (string literal could be
unchecked under NDEBUG)

- when using NDEBUG, assert exception cannot be evaluated. Previously
result was not used but non trivial assertions such as
  `container.find(key) != container.end()`
could be evaluated. More specifically functions with potential
boundary effects.

- also, this allows to compile the code with the `-pedantic` flag.