C11 standard adds the _Static_assert keyword.
This is implemented since gcc-4.6:
_Static_assert (0, "assert1"); /* { dg-error "static assertion failed: \"assert1\"" } */
The first slot needs to be an integral constant expression. The second slot is a constant string literal which can be long (_Static_assert(0, L"assertion of doom!")).
I should note that this is also implemented in recent versions of clang.