xkbvalidate: Don't rely on GNU extensions

The only reason why I was using _GNU_SOURCE was because of vasprintf(),
so getting rid of that extension should make the source way more
portable.

When using vsnprintf() with a null pointer for the output buffer and a
size of 0, I wasn't quite sure whether this would be undefined
behaviour, so I looked it up in the C11 standard.

In section 7.21.6.5, it explicitly mentions this case, so we're lucky:

  If n is zero, nothing is written, and s may be a null pointer.

Additionally, section 7.21.6.12 writes the following about vsnprintf():

  The vsnprintf function does not invoke the va_end macro.

So to be sure to avoid undefined behaviour I subsequently added the
corresponding va_end() calls.

With this, the platforms attribute is now "unix", because the program
should now even run on OS X, even though it usually wouldn't be needed.

Signed-off-by: aszlig <aszlig@nix.build>