This patch adds support for zero-length arrays in OpenACC data 
transfers. Previously, trying to use an array section with zero length 
would cause a fatal error at runtime.

This patch requires that my other patch "OpenACC: Separate enter/exit 
data APIs" is already applied.

Unfortunately, because the reference counting is handled by the code 
shared with OpenMP, and because the semantics there appear to be a 
little bit different (or broken?), I've been unable to get 
acc_is_present to return true for zero-length arrays created by pragmas 
(those created via acc_create are fine). That issue will require a 
another patch, probably with more invasive changes.

The test case should cover all the main uses of zero-length arrays, and 
I've added an xfail message to highlight the known deficiency.

OK for mainline (and backport to OG10)?

Andrew