use _Static_assert if we can
Submitted by Allison (desrt)
Link to original bug (#686773)
Description
It seem that GCC has a macro:
_Static_assert(2 + 2 == 5, "boom")
that does this:
xx.c:1:1: error: static assertion failed: "boom"
We should use this for G_STATIC_ASSERT, if we have it.